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Unsecured exchangeable senior notes

Mortgage notes payable

$ 2,813,338

$ 2,643,301

Unsecured senior notes

2,872,058

2,172,389

1,759,490

1,904,081

Net impact of adjustment for equity component allocation

104,419

$ 140,400

Carrying Amount [Member]

Unsecured exchangeable senior notes

Mortgage notes payable

2,813,338

Unsecured senior notes

2,872,058

^[1]

1,759,490

Long-term Debt, Total

7,444,886

Estimated of Fair Value [Member]

Unsecured exchangeable senior notes

Mortgage notes payable

2,912,476

Unsecured senior notes

3,316,303

1,973,879

Long-term Debt, Total

$ 8,202,658

[1]	Includes the unamortized net impact of the adjustment for the equity component allocation totaling approximately $104.4 million (See Note 7).

Real Estate Activity During the Nine Months Ended September 30, 2010 (Details) $ in Thousands	3 Months Ended		9 Months Ended		33 Months Ended
	Sep. 30, 2010 USD ($)	Sep. 30, 2009 USD ($)	Sep. 30, 2010 USD ($)	Sep. 30, 2009 USD ($)	Dec. 31, 2010 USD ($)	Dec. 31, 2009 USD ($)
Statement [Line Items]
Acquisition closing costs	$ 1,893		$ 1,893
Mortgage note payable assumed in connection with the acquisition of real estate			202,604
Mortgage loan	$ 2,813,338		2,813,338			$ 2,643,301
Gain on sale recognized		$ 2,394	$ 2,734	$ 9,682
250 West 55th Street in New York City [Member]
Statement [Line Items]
Office building, construction suspended, Sq feet	1,000,000		1,000,000
Costs accrued related to suspension of development			$ 27,800
Settlement of termination of the lease agreement	$ 12,800		12,800
Amount recognized due to termination of the lease agreement			$ 7,200
Weston Corporate Center in Weston Massachusetts [Member]
Statement [Line Items]
Weston Corporate Center, net rentable square feet	356,000		356,000
Weston Corporate Center, percentage leased			100.00%
Greater Washington, D.C. [Member]
Statement [Line Items]
Deferred gain on sale of parcel sold			$ 33,700
Net rentable square feet of development			165,000
Gain on sale recognized			$ 1,800		$ 23,400
280 Park Avenue [Member]
Statement [Line Items]
Gain on sale recognized			1,000
Management fees revenue			12,200
Reston Virginia [Member]
Statement [Line Items]
Acquisition costs	$ 20,300		20,300
Bay Colony Corporate Center [Member]
Statement [Line Items]
Acquisition costs	$ 185,000		$ 185,000
Maturity date			Jun. 11, 2012
Net Rentable Square Feet Of Acquisition	1,000,000		1,000,000
Business acquisition, date of acquisition agreement			font class="_mt"Septembernbsp;27, 2010/font
Cash portion of purchase price	$ 41,100		$ 41,100
Portion of acquisition included in debt	$ 143,900		$ 143,900
Interest on loan	6.53%		6.53%
Acquisition site, acres			58
510 Madison Avenue [Member]
Statement [Line Items]
Acquisition costs	$ 287,000		$ 287,000
Acquisition closing costs	1,500
Junior mezzanine loan interest acquisition cost			22,500
Mortgage note payable assumed in connection with the acquisition of real estate			202,600
Mortgage loan	$ 267,500		$ 267,500
Variable interest spread over LIBOR			0.30%
Maturity date			Feb. 24, 2012
Net Rentable Square Feet Of Acquisition	347,000		347,000
Contractual Amount Of Lease Agreement [Member] \| 250 West 55th Street in New York City [Member]
Statement [Line Items]
Costs accrued related to suspension of development				$ 20,000

Investments in Unconsolidated Joint Ventures (Investments in Unconsolidated Joint Ventures Table) (Details)

9 Months Ended

Sep. 30, 2010

Parcels of undeveloped land

0.67

Square 407 Limited Partnership [Member]

50.00%

The Metropolitan Square Associates LLC [Member]

Market Square North

51.00%

BP/CRF 901 New York Avenue LLC [Member]

Metropolitan Square

25.00%

^[1]

WP Project Developer LLC [Member]

901 New York Avenue

23.90%

^[2]

Wisconsin Place Retail LLC [Member]

Wisconsin Place Land and Infrastructure

5.00%

RBP Joint Venture LLC [Member]

Wisconsin Place Retail

50.00%

^[3]

Boston Properties Office Value Added Fund LP [Member]

Eighth Avenue and 46th Street

36.90%

^[1],[4]

Annapolis Junction NFM LLC [Member]

300 Billerica Road, One Two Circle Star Way and Mountain View Research and Technology Parks

50.00%

^[5]

Annapolis Junction

767 Venture LLC [Member]

60.00%

2 GCT Venture LLC [Member]

The General Motors Building

60.00%

540 Madison Venture LLC [Member]

Two Grand Central Tower

60.00%

125 West 55th Street Venture LLC [Member]

540 Madison Avenue

60.00%

500 North Capitol LLC [Member]

125 West 55th Street

30.00%

300 Billerica Road One Two Circle Star Way [Member]

500 North Capitol Street, NW

Mountain View Research And Technology Parks [Member]

25.00%

Entity Owning Land And Infrastructure Of Project [Member]

39.50%

Office Joint Venture Entities [Member]

33.33%

Retail Joint Venture Entities [Member]

66.67%

Residential Joint Venture Entities [Member]

5.00%

0.00%

[1]	The Company's economic ownership can increase based on the achievement of certain return thresholds.
[2]	Represents the Company's effective ownership interest. The Company has a 66.67%, 5% and 0% interest in the office, retail and residential joint venture entities, respectively, each of which owns a 33.33% interest in the entity owning the land and infrastructure of the project.
[3]	This property is not in operation and consists of assembled land.
[4]	Represents the Company's effective ownership interest. The Company has a 25.0% interest in the 300 Billerica Road and One amp; Two Circle Star Way properties and a 39.5% interest in the Mountain View Research and Technology Park properties (See also Note 15).
[5]	Two of the three Annapolis Junction land parcels are undeveloped land.

Investments in Unconsolidated Joint Ventures (Balance Sheets of the Unconsolidated Joint Ventures Table) (Details) - USD ($)
$ in Thousands

Sep. 30, 2010

Dec. 31, 2009

Real estate and development in process, net

$ 9,128,491

$ 8,658,495

Mortgage and notes payable

2,813,338

2,643,301

Other liabilities

104,914

127,538

Total liabilities and capital

12,647,617

11,941,317

Carrying value of the Company's investments in unconsolidated joint ventures

792,434

763,636

Unconsolidated Joint Ventures [Member]

Real estate and development in process, net

5,099,521

5,149,868

Other assets

806,595

760,001

Total assets

5,906,116

5,909,869

Mortgage and notes payable

3,257,015

3,217,893

Other liabilities

986,624

1,071,904

Members'/Partners' equity

1,662,477

1,620,072

Total liabilities and capital

5,906,116

5,909,869

Company's share of equity

948,064

927,184

Basis differentials

^[1]

(155,630)

(163,548)

Carrying value of the Company's investments in unconsolidated joint ventures

$ 792,434

$ 763,636

[1]

This amount represents the aggregate difference between the Company's historical cost basis and the basis reflected at the joint venture level, which is typically amortized over the life of the related assets and liabilities. Basis differentials occur from impairment of investments and upon the transfer of assets that were previously owned by the Company into a joint venture. In addition, certain acquisition, transaction and other costs may not be reflected in the net assets at the joint venture level.

Investments in Unconsolidated Joint Ventures (Statements of Operations of the Joint Ventures Table) (Details) - USD ($)
$ in Thousands

3 Months Ended

9 Months Ended

Sep. 30, 2010

Sep. 30, 2009

Sep. 30, 2010

Sep. 30, 2009

Joint ventures total revenue

Total revenue

$ 371,955

$ 359,386

$ 1,101,765

$ 1,095,353

388,224

377,303

1,163,963

1,144,337

Interest

97,103

77,090

285,887

234,653

Depreciation and amortization

79,077

76,126

239,441

236,389

Losses from early extinguishments of debt

(16)

(8,221)

(510)

Total expenses

329,644

306,342

984,387

947,195

Net income

70,145

79,705

209,250

217,920

Income from unconsolidated joint ventures

11,565

6,350

26,940

11,096

Unconsolidated Joint Ventures [Member]

Depreciation and amortization

Total revenue

^[1]

155,126

148,885

457,217

445,827

Operating

45,546

41,977

130,083

122,614

Interest

59,120

58,975

176,705

173,356

50,548

57,160

164,763

175,648

Other

3,581

Total expenses

158,795

158,112

475,132

471,618

Net income

(3,669)

(9,227)

(17,915)

(25,791)

Company's share of net loss

(487)

(4,174)

(7,207)

(12,619)

Impairment loss on investments

(7,357)

Basis differential

3,125

2,319

7,918

7,099

Elimination of inter-entity interest on partner loan

8,927

8,205

26,229

23,973

Income from unconsolidated joint ventures

$ 11,565

$ 6,350

$ 26,940

$ 11,096

[1]

Includes straight-line rent adjustments of $5.9 million and $6.7 million for the three months ended September 30, 2010 and 2009, respectively, and $19.8 million and $20.3 million for the nine months ended September 30, 2010 and 2009, respectively. Includes net "above-" and "below-market" rent adjustments of $30.5 million and $37.8 million for the three months ended September 30, 2010 and 2009, respectively, and $97.6 million and $115.1 million for the nine months ended September 30, 2010 and 2009, respectively.

Investments in Unconsolidated Joint Ventures (Statements of Operations of the Joint Ventures) (Parenthetical) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2010	Sep. 30, 2009	Sep. 30, 2010	Sep. 30, 2009
Investments in Unconsolidated Joint Ventures
Straight-line rent adjustments	$ 5.9	$ 6.7	$ 19.8	$ 20.3
Above and below market rent adjustments, net	$ 30.5	$ 37.8	$ 97.6	$ 115.1

Investments in Unconsolidated Joint Ventures (Details)

9 Months Ended

Sep. 30, 2010

USD ($)

The Metropolitan Square Associates LLC [Member]

The Metropolitan Square Associates LLC [Member] | Previous Mortgage Loan [Member]

51.00%

Mortgage loan, carrying amount

$ 123,600,000

8.23%

The Metropolitan Square Associates LLC [Member] | New Mortgage Loan [Member]

May 01, 2010

5.75%

May 05, 2020

Annapolis Junction NFM LLC [Member]

$ 175,000,000

Mortgage loan, carrying amount

50.00%

^[1]

$ 42,700,000

Mortgage loan extension option, years

Sep. 11, 2011

Variable interest spread over LIBOR

1.00%

2 GCT Venture LLC [Member]

2 GCT Venture LLC [Member] | Previous Mortgage Loan [Member]

60.00%

$ 190,000,000

5.10%

Repayment of previous mortgage by investor

Jul. 11, 2010

$ 10,000,000

2 GCT Venture LLC [Member] | New Mortgage Loan [Member]

$ 180,000,000

6.00%

125 West 55th Street Venture LLC [Member]

Apr. 10, 2015

Guarantee maximum funding obligation

60.00%

$ 21,300,000

Cash deposit into the escrow account

10,000,000

Escrow account remaining funding obligation

11,300,000

Maximum exposure as of period end

$ 8,600,000

125 West 55th Street Venture LLC [Member] | Mezzanine Loans [Member]

7.81%

Repayment outstanding mezzanine loans

Mar. 01, 2010

$ 63,500,000

125 West 55th Street Venture LLC [Member] | New Mortgage Loan [Member]

Guarantee maximum funding obligation

21,300,000

Escrow account remaining funding obligation

11,300,000

Maximum exposure as of period end

$ 8,600,000

500 North Capitol LLC [Member]

North Capital Street net rentable square feet

30.00%

176,000

Investment in joint venture, total cost

$ 1,900,000

Investments in joint venture, cash contributions

$ 1,400,000

Investments in joint venture common units issued | shares

5,906

500 North Capitol LLC [Member] | Previous Mortgage Loan [Member]

500 North Capitol LLC [Member] | New Mortgage Loan [Member]

$ 26,800,000

Variable interest rate at period end

Mar. 31, 2013

5.75%

Eighth Avenue and 46th Street New York [Member]

$ 22,000,000

Land parcels and development rights received in exchange

50.00%

$ 6,400,000

Land parcels and development rights relinquished in exchange

5,400,000

Payment from land parcels and development rights

$ 1,000,000

Mortgage repayment date

Jun. 15, 2010

Market Square [Member]

Market Square [Member] | Previous Mortgage Loan [Member]

50.00%

Mortgage loan, carrying amount

$ 81,100,000

7.70%

Market Square [Member] | New Mortgage Loan [Member]

Dec. 19, 2010

4.85%

Oct. 01, 2020

Joint Venture Which Modified Mortgage [Member] | 125 West 55th Street Venture LLC [Member] | New Mortgage Loan [Member]

$ 130,000,000

Joint Venture Which Refinanced Mortgage [Member] | 125 West 55th Street Venture LLC [Member]

Mar. 10, 2020

Joint Venture Which Refinanced Mortgage [Member] | 125 West 55th Street Venture LLC [Member] | Previous Mortgage Loan [Member]

60.00%

$ 200,000,000

5.75%

Joint Venture Which Refinanced Mortgage [Member] | 125 West 55th Street Venture LLC [Member] | New Mortgage Loan [Member]

Mar. 01, 2010

6.09%

Mar. 10, 2015

Cash deposit into the escrow account

$ 207,000,000

$ 10,000,000

[1]	Two of the three Annapolis Junction land parcels are undeveloped land.

Mortgage Notes Payable (Details) - USD ($) $ in Thousands	9 Months Ended
Mortgage Notes Payable (Details) - USD ($) $ in Thousands	Sep. 30, 2010	Sep. 30, 2009	Dec. 31, 2009
Statement [Line Items]
Mortgage loan related to property sales	$ 301,531	$ 119,268
Mortgage note payable assumed in connection with the acquisition of real estate	202,604
Mortgage loan	2,813,338		$ 2,643,301
Cambridge Center [Member]
Statement [Line Items]
Mortgage loan related to property sales	$ 22,600
Interest on loan	7.73%
Maturity date	Jul. 15, 2010
Pre-payment penalty	$ 0
Carnegie Center 202 206 & 214 [Member]
Statement [Line Items]
Mortgage loan related to property sales	$ 55,800
Interest on loan	8.13%
Maturity date	Oct. 01, 2010
Pre-payment penalty	$ 0
Reservoir Place [Member]
Statement [Line Items]
Mortgage loan, carrying amount	$ 50,000
Variable interest spread over LIBOR	3.85%
Variable interest spread over Eurodollar	2.20%
Maturity date	Jul. 30, 2014
510 Madison Avenue [Member]
Statement [Line Items]
Variable interest spread over LIBOR	0.30%
Maturity date	Feb. 24, 2012
Mortgage note payable assumed in connection with the acquisition of real estate	$ 202,600
Mortgage loan	$ 267,500

Unsecured Senior Notes (Unsecured Senior Notes Outstanding Table) (Details) - USD ($)
$ in Thousands

9 Months Ended

Sep. 30, 2010

Dec. 31, 2009

Amount due prior to maturity

$ 1,873,694

Net discount

(2,942)

$ (2,611)

Senior Notes, Total

2,872,058

$ 2,172,389

Senior Notes [Member]

10 Year Unsecured Senior Notes 6.381% [Member]

2,875,000

Net discount

$ (2,942)

6.25%

^[1]

6.381%

$ 750,000

10 Year Unsecured Senior Notes 6.291% [Member]

^[2]

Jan. 15, 2013

6.25%

^[1]

6.291%

$ 175,000

12 Year Unsecured Senior Notes 5.693% [Member]

^[2]

Jan. 15, 2013

5.625%

^[1]

5.693%

$ 300,000

12 Year Unsecured Senior Notes 5.194% [Member]

^[2]

Apr. 15, 2015

5.00%

^[1]

5.194%

$ 250,000

10 Year Unsecured Senior Notes 5.967% [Member]

^[2]

Jun. 01, 2015

5.875%

^[1]

5.967%

$ 700,000

10 Year Unsecured Senior Notes 5.708% [Member]

^[2]

Oct. 15, 2019

5.625%

^[1]

5.708%

$ 700,000

^[2]

Nov. 15, 2020

[1]	Yield on issuance date including the effects of discounts on the notes.
[2]	No principal amounts are due prior to maturity.

Unsecured Senior Notes (Narratives) (Details)	9 Months Ended
Unsecured Senior Notes (Narratives) (Details)	Sep. 30, 2010 USD ($)
Statement [Line Items]
Net proceed of senior note after offering expense	$ 699,237,000
Treasury rate lock agreement rate basis, treasury term	10
Fixed treasury interest rate ten year	3.873%
Notional amount of treasury rate lock	$ 350,000,000
Cash-settled treasury rate lock agreements	$ 400,000
Number of treasury lock agreements	2
Senior Notes 5.625% Due 2020 [Member]
Statement [Line Items]
Long-term debt, percentage bearing fixed interest rate	5.625%
Net proceed of senior note after offering expense	$ 693,500,000
Public offering date	April 19, 2010
Aggregate principal amount	$ 700,000,000
Maturity date	Nov. 15, 2020
Note price, percentage of principal amount	99.891%
Yield	5.708%

Unsecured Exchangeable Senior Notes (Unsecured Exchangeable Senior Notes Outstanding) (Details)

9 Months Ended

12 Months Ended

Sep. 30, 2010

USD ($)

$ / shares

Dec. 31, 2007

USD ($)

$ / shares

Dec. 17, 2007

$ / shares

Dec. 31, 2009

USD ($)

Aug. 13, 2008

$ / shares

Net impact of adjustment for equity component allocation

$ 1,873,694,000

(104,419,000)

$ (140,400,000)

Unsecured Exchangeable Senior Notes Discount

(9,785,000)

(15,529,000)

Total principal amount of Unsecured Exchangeable Senior Notes

$ 1,759,490,000

$ 1,904,081,000

Revised exchange price due to capped call transaction | $ / shares

$ 136.13

3.625% Exchangeable Senior Notes [Member]

3.625%

Effective rate

^[1]

4.037%

^[2]

8.5051

First Optional Redemption Date

$ 747,500,000

N/A

Initial exchange per share

Feb. 15, 2014

Initial exchange price

$ 1,000

$ 117.58

Revised exchange price due to capped call transaction | $ / shares

$ 137.17

Premium over closing price due to capped call

40.00%

Closing price of Boston Properties, Inc.'s common stock per share | $ / shares

$ 97.98

Net cost of capped call transaction

$ 44,400,000

2.875% Exchangeable Senior Notes [Member]

2.875%

Effective rate

^[1]

3.462%

^[3]

7.0430

First Optional Redemption Date

$ 676,194,000

^[4]

February 20, 2012(4)

Special distribution per share of common stock | $ / shares

Feb. 15, 2037

Initial exchange price

$ 1,000

$ 5.98

Initial exchange rate

6.6090

Revised exchange rate

7.0430

Revised exchange price | $ / shares

$ 141.98

Repurchase price for the notes, percentage of principal

100.00%

3.750% Exchangeable Senior Notes [Member]

3.75%

Effective rate

^[1]

3.787%

^[5]

10.0066

First Optional Redemption Date

$ 450,000,000

^[6]

May 18, 2013(6)

Special distribution per share of common stock | $ / shares

May 15, 2036

Initial exchange price

$ 1,000

$ 5.98

Revised exchange price | $ / shares

$ 99.93

Repurchase price for the notes, percentage of principal

100.00%

3.750% Exchangeable Senior Notes [Member]

Initial exchange rate

9.3900

Revised exchange rate

10.0066

[1]	Yield on issuance date including the effects of discounts on the notes but excluding the effects of the adjustment for the equity component allocation.
[2]	The initial exchange rate is 8.5051 shares per $1,000 principal amount of the notes (or an initial exchange price of approximately $117.58 per share of Boston Properties, Inc.'s common stock). In addition, the Company entered into capped call transactions with affiliates of certain of the initial purchasers, which are intended to reduce the potential dilution upon future exchange of the notes. The capped call transactions were intended to increase the effective exchange price to the Company of the notes from $117.58 to approximately $137.17 per share (subject to adjustment), representing an overall effective premium of approximately 40% over the closing price on August 13, 2008 of $97.98 per share of Boston Properties, Inc.'s common stock. The net cost of the capped call transactions was approximately $44.4 million. As of September 30, 2010, the effective exchange price was $136.13 per share.
[3]	In connection with the special distribution of $5.98 per share of Boston Properties, Inc.'s common stock declared on December 17, 2007, the exchange rate was adjusted from 6.6090 to 7.0430 shares per $1,000 principal amount of notes effective as of December 31, 2007, resulting in an exchange price of approximately $141.98 per share of Boston Properties, Inc.'s common stock.
[4]	Holders may require the Company to repurchase the notes for cash on February 15, 2012, 2017, 2022, 2027 and 2032 and at any time prior to their maturity upon a fundamental change, in each case at a price equal to 100% of the principal amount of the notes being repurchased plus any accrued and unpaid interest up to, but excluding, the repurchase date.
[5]	In connection with the special distribution of $5.98 per share of Boston Properties, Inc.'s common stock declared on December 17, 2007, the exchange rate was adjusted from 9.3900 to 10.0066 shares per $1,000 principal amount of notes effective as of December 31, 2007, resulting in an exchange price of approximately $99.93 per share of Boston Properties, Inc.'s common stock.
[6]	Holders may require the Company to repurchase the notes for cash on May 18, 2013 and May 15, 2016, 2021, 2026 and 2031 and at any time prior to their maturity upon a fundamental change, in each case at a price equal to 100% of the principal amount of the notes being repurchased plus any accrued and unpaid interest up to, but excluding, the repurchase date.

Unsecured Exchangeable Senior Notes (Narratives) (Details) - USD ($)
$ in Thousands

3 Months Ended

9 Months Ended

Sep. 30, 2010

Sep. 30, 2009

Sep. 30, 2010

Sep. 30, 2009

Dec. 31, 2009

Net impact of adjustment for equity component allocation

$ 104,419

$ 140,400

Proceeds attributable to equity component of the notes

230,300

Deferred financing costs attributable to equity component

1,000

Interest expense

17,100

$ 18,600

52,300

$ 55,800

Non-cash interest expense

9,500

9,800

28,800

28,700

Carrying amount of equity component

229,300

Losses from early extinguishments of debt

$ 16

8,221

$ 510

Aggregate carrying value

1,759,490

$ 1,904,081

Amount paid for repurchase

172,294

3.625% Exchangeable Senior Notes [Member]

Value of exchangeable senior notes

$ 747,500

^[1]

8.5051

Debt instrument effective interest rate

3.625%

6.555%

2.875% Exchangeable Senior Notes [Member]

Feb. 15, 2014

Value of exchangeable senior notes

$ 862,500

^[2]

7.0430

Debt instrument effective interest rate

2.875%

5.63%

Losses from early extinguishments of debt

Feb. 15, 2037

$ 8,200

Amount paid for repurchase

185,500

Value of repurchase of senior notes due in 2037

186,300

Carrying value of repurchased notes

177,300

3.750% Exchangeable Senior Notes [Member]

Value of exchangeable senior notes

$ 450,000

^[3]

10.0066

Debt instrument effective interest rate

3.75%

5.958%