Boston Properties (BXP) 10-K 2 Mar 15 2014 FY Annual report Financial data

Document_And_Entity_Informatio

Document And Entity Information (USD $)	12 Months Ended
	Dec. 31, 2014	Feb. 23, 2015	Jun. 30, 2014
Document And Entity Information [Abstract]
Document Type	10-K
Document Period End Date	31-Dec-14
Document Fiscal Year Focus	2014
Document Fiscal Period Focus	FY
Entity Registrant Name	BOSTON PROPERTIES INC
Amendment Flag	FALSE
Entity Central Index Key	1037540
Current Fiscal Year End Date	-19
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		153,187,903
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Public Float			$17,955,016,728

Consolidated_Balance_Sheets

Consolidated Balance Sheets (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
ASSETS
Real estate, at cost	$19,236,403	$18,978,765
Less: accumulated depreciation	-3,547,659	-3,161,571
Total real estate	15,688,744	15,817,194
Cash and cash equivalents	1,763,079	2,365,137
Cash held in escrows	487,321	57,201
Investments in securities	19,459	16,641
Tenant and other receivables (net of allowance for doubtful accounts of $1,142 and $1,636, respectively)	46,595	59,464
Accrued rental income (net of allowance of $1,499 and $3,636, respectively)	691,999	651,603
Deferred charges, net	831,744	884,450
Prepaid expenses and other assets	164,432	184,477
Investments in unconsolidated joint ventures	193,394	140,097
Total assets	19,886,767	20,176,264
Liabilities:
Mortgage notes payable	4,309,484	4,449,734
Unsecured senior notes (net of discount of $12,296 and $14,146, respectively)	5,287,704	5,835,854
Unsecured exchangeable senior notes (net of discount of $0 and $182, respectively)	0	744,880
Unsecured line of credit	0	0
Mezzanine notes payable	309,796	311,040
Outside members' notes payable	180,000	180,000
Accounts payable and accrued expenses	243,263	202,470
Dividends and distributions payable	882,472	497,242
Accrued interest payable	163,532	167,523
Other liabilities	502,255	592,982
Total liabilities	11,878,506	12,981,725
Commitments and contingencies	0	0
Noncontrolling interest:
Redeemable preferred units of the Operating Partnership	633	51,312
Redeemable interest in property partnership	104,692	99,609
Equity:
Excess stock, $.01 par value, 150,000,000 shares authorized, none issued or outstanding	0	0
Preferred stock, $.01 par value, 50,000,000 shares authorized;
5.25% Series B cumulative redeemable preferred stock, $.01 par value, liquidation preference $2,500 per share, 92,000 shares authorized, 80,000 shares issued and outstanding at December 31, 2014 and December 31, 2013, respectively	200,000	200,000
Common stock, $.01 par value, 250,000,000 shares authorized, 153,192,845 and 153,062,001 issued and 153,113,945 and 152,983,101 outstanding at December 31, 2014 and December 31, 2013, respectively	1,531	1,530
Additional paid-in capital	6,270,257	5,662,453
Dividends in excess of earnings	-762,464	-108,552
Treasury common stock at cost, 78,900 shares at December 31, 2014 and December 31, 2013	-2,722	-2,722
Accumulated other comprehensive loss	-9,304	-11,556
Total stockholders’ equity attributable to Boston Properties, Inc.	5,697,298	5,741,153
Noncontrolling interests:
Common units of the Operating Partnership	603,171	576,333
Property partnerships	1,602,467	726,132
Total equity	7,902,936	7,043,618
Total liabilities and equity	$19,886,767	$20,176,264

Consolidated_Balance_Sheets_Pa

Consolidated Balance Sheets (Parenthetical) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, except Share data, unless otherwise specified
Tenant and other receivables, allowance for doubtful accounts	$1,142	$1,636
Accrued rental income, allowance	1,499	3,636
Unsecured senior notes, discount	12,296	14,146
Unsecured exchangeable senior notes, discount	$0	$182
Excess stock, par value	$0.01	$0.01
Excess stock, shares authorized	150,000,000	150,000,000
Excess stock, shares issued	0	0
Excess stock, shares outstanding	0	0
Preferred stock, par value	$0.01	$0.01
Preferred stock, shares authorized	50,000,000	50,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$0.01	$0.01
Common stock, shares authorized	250,000,000	250,000,000
Common stock, shares issued	153,192,845	153,062,001
Common stock, shares outstanding	153,113,945	152,983,101
Treasury common stock at cost, shares	78,900	78,900
Series B Cumulative Redeemable Preferred Stock [Member]
Preferred stock, par value	$0.01	$0.01
Preferred stock, shares authorized	92,000	92,000
Series B Dividend Rate Percentage	5.25%	5.25%
Preferred stock, shares issued	80,000	80,000
Series B Liquidation Preference Per Share	$2,500	$2,500
Preferred stock, shares outstanding	80,000	80,000

Consolidated_Statements_Of_Ope

Consolidated Statements Of Operations (USD $)	12 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Revenue
Base rent	$1,886,339	$1,675,412	$1,457,834
Recoveries from tenants	339,365	292,944	228,170
Parking and other	102,593	97,158	89,207
Total rental revenue	2,328,297	2,065,514	1,775,211
Hotel revenue	43,385	40,330	37,915
Development and management services	25,316	29,695	34,060
Total revenue	2,396,998	2,135,539	1,847,186
Expenses
Rental	835,290	742,956	639,088
Hotel	29,236	28,447	28,120
General and administrative	98,937	115,329	90,129
Transaction costs	3,140	1,744	3,653
Impairment loss	0	8,306	0
Depreciation and amortization	628,573	560,637	445,875
Total expenses	1,595,176	1,457,419	1,206,865
Operating income	801,822	678,120	640,321
Other income (expense)
Income from unconsolidated joint ventures	12,769	75,074	49,078
Gains on consolidation of joint ventures	0	385,991	0
Interest and other income	8,765	8,310	10,091
Gains from investments in securities	1,038	2,911	1,389
Interest expense	-455,743	-446,880	-410,970
(Losses) gains from early extinguishments of debt	-10,633	122	-4,453
Income from continuing operations	358,018	703,648	285,456
Discontinued operations
Income from discontinued operations	0	8,022	9,806
Gains on sales of real estate from discontinued operations	0	112,829	36,877
Gain on forgiveness of debt from discontinued operations	0	20,182	0
Impairment loss from discontinued operations	0	-3,241	0
Income before gains on sales of real estate	358,018	841,440	332,139
Gains on sales of real estate	168,039	0	0
Net income	526,057	841,440	332,139
Net income attributable to noncontrolling interests
Noncontrolling interests in property partnerships	-30,561	-1,347	-3,792
Noncontrolling interest—redeemable preferred units of the Operating Partnership	-1,023	-6,046	-3,497
Noncontrolling interest—common units of the Operating Partnership	-50,862	-70,085	-30,125
Noncontrolling interest in discontinued operations—common units of the Operating Partnership	0	-14,151	-5,075
Net income attributable to Boston Properties, Inc.	443,611	749,811	289,650
Preferred dividends	-10,500	-8,057	0
Net income attributable to Boston Properties, Inc. common shareholders	$433,111	$741,754	$289,650
Basic earnings per common share attributable to Boston Properties, Inc.:
Income from continuing operations	$2.83	$4.06	$1.65
Discontinued operations	$0	$0.81	$0.28
Net income	$2.83	$4.87	$1.93
Weighted average number of common shares outstanding	153,089	152,201	150,120
Diluted earnings per common share attributable to Boston Properties, Inc.:
Income from continuing operations	$2.83	$4.05	$1.64
Discontinued operations	$0	$0.81	$0.28
Net income	$2.83	$4.86	$1.92
Weighted average number of common and common equivalent shares outstanding	153,308	152,521	150,711

Consolidated_Statements_Of_Com

Consolidated Statements Of Comprehensive Income (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Statement of Comprehensive Income [Abstract]
Net income	$526,057		$841,440		$332,139
Other comprehensive income:
Amortization of interest rate contracts (1)	2,508	[1]	2,513	[1]	2,594	[1]
Other comprehensive income	2,508		2,513		2,594
Comprehensive income	528,565		843,953		334,733
Net income attributable to noncontrolling interests	-82,446		-91,629		-42,489
Other comprehensive income attributable to noncontrolling interests	-256		-252		-273
Comprehensive income attributable to Boston Properties, Inc.	$445,863		$752,072		$291,971
[1]	Amounts reclassified from comprehensive income primarily to interest expense within the Company's Consolidated Statements of Operations

Consolidated_Statements_Of_Sto

Consolidated Statements Of Stockholders' Equity (USD $)	Total	Common Stock [Member]	Preferred Stock [Member]	Additional Paid-In Capital [Member]	Dividends In Excess Of Earnings [Member]	Treasury Stock, At Cost [Member]	Accumulated Other Comprehensive Loss [Member]	Noncontrolling Interests [Member]
In Thousands, except Share data, unless otherwise specified
Equity, value at Dec. 31, 2011	$5,413,516	$1,481		$4,936,457	($53,080)	($2,722)	($16,138)	$547,518
Equity, shares at Dec. 31, 2011		148,108,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Conversion of operating partnership units to Common Stock, shares		1,111,000
Conversion of operating partnership units to Common Stock, value	0	11		34,610				-34,621
Conversion of redeemable preferred units to common units	5,852							5,852
Allocated net income for the year	326,839				289,650			37,189
Dividends/distributions declared	-387,989				-346,555			-41,434
Shares issued in connection with exchange of exchangeable senior notes, value	0
Sale of Common Stock, net of offering costs, shares		2,348,000
Sale of Common Stock, net of offering costs, value	247,027	24		247,003
Shares issued pursuant to stock purchase plan, shares		7,000
Shares issued pursuant to stock purchase plan, value	781			781
Net activity from stock option and incentive plan, shares		27,000
Net activity from stock option and incentive plan, value	29,124			5,419				23,705
Noncontrolling interest in property partnerships recorded upon consolidation	0
Distributions to noncontrolling interests in property partnerships	-2,890							-2,890
Amortization of interest rate contracts	2,594						2,321	273
Reallocation of noncontrolling interest	0			-2,197				2,197
Equity, value at Dec. 31, 2012	5,634,854	1,516		5,222,073	-109,985	-2,722	-13,817	537,789
Equity, shares at Dec. 31, 2012		151,601,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Conversion of operating partnership units to Common Stock, shares		929,000
Conversion of operating partnership units to Common Stock, value	0	10		30,281				-30,291
Conversion of redeemable preferred units to common units	16,494							16,494
Allocated net income for the year	828,757				749,811			78,946
Dividends/distributions declared	-831,826				-748,378			-83,448
Issuance of 5.25% Series B cumulative redeemable preferred stock	193,623		200,000	-6,377
Shares issued in connection with exchange of exchangeable senior notes, value	43,834	4		43,830
Equity component of exchange of exchangeable senior notes	-43,869			-43,869
shares issued in connection with exchange of exchangeable senior notes, shares		419,000
Shares issued pursuant to stock purchase plan, shares		6,000
Shares issued pursuant to stock purchase plan, value	681			681
Net activity from stock option and incentive plan, shares		28,000
Net activity from stock option and incentive plan, value	35,571			7,701				27,870
Noncontrolling interest in property partnerships recorded upon consolidation	480,861							480,861
Sale of interest in property partnerships and contributions from noncontrolling interest in property partnerships	687,164			429,600				257,564
Distributions to noncontrolling interests in property partnerships	-5,039							-5,039
Amortization of interest rate contracts	2,513						2,261	252
Reallocation of noncontrolling interest	0			-21,467				21,467
Equity, value at Dec. 31, 2013	7,043,618	1,530	200,000	5,662,453	-108,552	-2,722	-11,556	1,302,465
Equity, shares at Dec. 31, 2013	152,983,101	152,983,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Conversion of operating partnership units to Common Stock, shares		80,000
Conversion of operating partnership units to Common Stock, value	0	1		2,699				-2,700
Conversion of redeemable preferred units to common units	33,306							33,306
Allocated net income for the year	513,951				443,611			70,340
Dividends/distributions declared	-1,224,471				-1,097,523			-126,948
Shares issued in connection with exchange of exchangeable senior notes, value	0
Shares issued pursuant to stock purchase plan, shares		7,000
Shares issued pursuant to stock purchase plan, value	761			761
Net activity from stock option and incentive plan, shares		44,000
Net activity from stock option and incentive plan, value	27,999			6,822				21,177
Noncontrolling interest in property partnerships recorded upon consolidation	0
Sale of interest in property partnerships and contributions from noncontrolling interest in property partnerships	1,536,382			648,407				887,975
Distributions to noncontrolling interests in property partnerships	-31,118							-31,118
Amortization of interest rate contracts	2,508						2,252	256
Reallocation of noncontrolling interest	0			-50,885				50,885
Equity, value at Dec. 31, 2014	$7,902,936	$1,531	$200,000	$6,270,257	($762,464)	($2,722)	($9,304)	$2,205,638
Equity, shares at Dec. 31, 2014	153,113,945	153,114,000

Consolidated_Statements_Of_Cas

Consolidated Statements Of Cash Flows (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Cash flows from operating activities:
Net income	$526,057	$841,440	$332,139
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	628,573	565,397	454,044
Non-cash compensation expense	28,099	45,155	29,679
Impairment loss	0	8,306	0
Income from unconsolidated joint ventures	-12,769	-75,074	-49,078
Gains on consolidation of joint ventures	0	385,991	0
Distributions of net cash flow from operations of unconsolidated joint ventures	7,372	32,600	47,002
Gains from investments in securities	-1,038	-2,911	-1,389
Non-cash portion of interest expense	-39,343	2,649	43,131
Settlement of accreted debt discount on repurchases/repayments of unsecured senior notes and unsecured exchangeable senior notes	-94,963	-56,532	-69,499
Losses (gains) from early extinguishments of debt	0	-264	-1,000
Gains on sales of real estate	-168,039	0	0
Gains on sales of real estate from discontinued operations	0	-112,829	-36,877
Gain on forgiveness of debt from discontinued operations	0	-20,182	0
Impairment loss from discontinued operations	0	3,241	0
Change in assets and liabilities:
Cash held in escrows	3,433	315	10,272
Tenant and other receivables, net	12,869	-443	23,155
Accrued rental income, net	-57,899	-59,972	-77,363
Prepaid expenses and other assets	20,238	12,966	6,990
Accounts payable and accrued expenses	3,903	13,108	3,854
Accrued interest payable	-3,991	21,302	3,356
Other liabilities	-57,873	2,073	1,354
Tenant leasing costs	-99,076	-56,428	-76,821
Total adjustments	169,496	-63,514	310,810
Net cash provided by operating activities	695,553	777,926	642,949
Cash flows from investing activities:
Acquisitions of real estate	-4,670	-522,900	-788,052
Construction in progress	-405,942	-396,835	-356,397
Building and other capital improvements	-82,479	-73,821	-49,943
Tenant improvements	-106,003	-105,425	-139,662
Proceeds from sales of real estate	419,864	250,078	61,963
Proceeds from sales of real estate and sales of interests in property partnerships placed in escrow	-1,912,347	0	0
Proceeds from sales of real estate and sales of interests in property partnerships released from escrow	1,478,794	0	0
Cash recorded upon consolidation	0	79,468	0
Issuance of notes receivable, net	0	12,491	-2,049
Capital contributions to unconsolidated joint ventures	-52,052	0	-6,214
Capital distributions from unconsolidated joint ventures	1,491	225,862	3,557
Investments in securities, net	-1,780	-1,558	-1,235
Net cash used in investing activities	-665,124	-532,640	-1,278,032
Cash flows from financing activities:
Repayments of mortgage notes payable	-87,758	-80,311	-253,877
Proceeds from unsecured senior notes	0	1,194,753	997,790
Redemption/repurchase of unsecured senior notes	-548,016	0	-224,261
Redemption/repurchase of unsecured exchangeable senior notes	-654,521	-393,468	-507,434
Proceeds from real estate transaction	14,523	0	0
Payments on real estate transaction	-234	0	0
Deferred financing costs	-31	-15,195	-8,468
Net proceeds from preferred stock issuance	0	193,623	0
Net proceeds from ATM stock issuances	0	0	247,027
Net proceeds from equity transactions	1,923	-334	226
Redemption of preferred units	-17,373	-43,070	-18,329
Dividends and distributions	-840,264	-451,118	-372,899
Sales of interests in property partnerships and contributions from noncontrolling interests in property partnerships	1,536,382	682,617	0
Distributions to noncontrolling interests in property partnerships	-37,118	-9,624	-5,922
Net cash provided by (used in) financing activities	-632,487	1,077,873	-146,147
Net increase (decrease) in cash and cash equivalents	-602,058	1,323,159	-781,230
Cash and cash equivalents, beginning of year	2,365,137	1,041,978	1,823,208
Cash and cash equivalents, end of year	1,763,079	2,365,137	1,041,978
Supplemental disclosures:
Cash paid for interest	646,516	547,973	480,866
Interest capitalized	52,476	68,152	44,278
Non-cash investing and financing activities:
Change in real estate included in accounts payable and accrued expenses	-1,431	-19,824	14,059
Real estate and related intangibles recorded upon consolidation	0	3,356,000	0
Debt recorded upon consolidation	0	2,056,000	0
Working capital recorded upon consolidation	0	177,315	0
Noncontrolling interests recorded upon consolidation	0	480,861	0
Investment in unconsolidated joint venture eliminated upon consolidation	0	361,808	0
Mortgage note payable extinguished through foreclosure	0	25,000	0
Real estate transferred upon foreclosure	0	7,508	0
Land improvements contributed by noncontrolling interest in property partnership	0	4,546	0
Mortgage note payable assumed in connection with the acquisition of real estate	0	0	211,250
Redeemable noncontrolling interest in property partnership	0	0	98,787
Preferred units issued in connection with the acquisition of real estate	0	0	79,405
Dividends and distributions declared but not paid	882,472	497,242	110,488
Issuance of common stock in connection with the exchange of exchangeable senior notes	0	43,834	0
Conversions of noncontrolling interests to stockholders’ equity	2,700	30,291	34,621
Conversion of redeemable preferred units to common units	33,306	16,494	5,852
Issuance of restricted securities to employees and directors	$27,445	$30,077	$26,198

Organization

Organization	12 Months Ended
	Dec. 31, 2014
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Organization	Organization
	Boston Properties, Inc. (the “Company”), a Delaware corporation, is a self-administered and self-managed real estate investment trust (“REIT”). The Company is the sole general partner of Boston Properties Limited Partnership (the “Operating Partnership”) and at December 31, 2014 owned an approximate 89.5% (89.5% at December 31, 2013) general and limited partnership interest in the Operating Partnership. Partnership interests in the Operating Partnership are denominated as “common units of partnership interest” (also referred to as “OP Units”), “long term incentive units of partnership interest” (also referred to as “LTIP Units”) or “preferred units of partnership interest” (also referred to as “Preferred Units”). In addition, in February 2011 and February 2012, the Company issued LTIP Units in connection with the granting to employees of outperformance awards (also referred to as “2011 OPP Units” and “2012 OPP Units,” respectively, and collectively as “OPP Units”) (See Note 20). On January 31, 2014, the measurement period for the Company’s 2011 OPP Unit awards expired and the Company’s total return to shareholders (“TRS”) was not sufficient for employees to earn and therefore become eligible to vest in any of the 2011 OPP Unit awards. Accordingly, all 2011 OPP Unit awards were automatically forfeited (See Notes 11 and 17). In February 2013 and February 2014, the Company issued LTIP Units in connection with the granting to employees of multi-year, long-term incentive program ("MYLTIP") awards (also referred to as “2013 MYLTIP Units” and “2014 MYLTIP Units,” respectively, and collectively as "MYLTIP Units"). Because the rights, preferences and privileges of OPP Units and MYLTIP Units differ from other LTIP Units granted to employees as part of the annual compensation process, unless specifically noted otherwise, all references to LTIP Units exclude OPP Units and MYLTIP Units (See Notes 11 and 17).
	Unless specifically noted otherwise, all references to OP Units exclude units held by the Company. A holder of an OP Unit may present such OP Unit to the Operating Partnership for redemption at any time (subject to restrictions agreed upon at the time of issuance of OP Units to particular holders that may restrict such redemption right for a period of time, generally one year from issuance). Upon presentation of an OP Unit for redemption, the Operating Partnership is obligated to redeem such OP Unit for cash equal to the value of a share of common stock of the Company (“Common Stock”) at such time. In lieu of a cash redemption, the Company may elect to acquire such OP Unit for one share of Common Stock. Because the number of shares of Common Stock outstanding at all times equals the number of OP Units that the Company owns, one share of Common Stock is generally the economic equivalent of one OP Unit, and the quarterly distribution that may be paid to the holder of an OP Unit equals the quarterly dividend that may be paid to the holder of a share of Common Stock. An LTIP Unit is generally the economic equivalent of a share of restricted common stock of the Company. LTIP Units, whether vested or not, will receive the same quarterly per unit distributions as OP Units, which equal per share dividends on Common Stock (See Note 12).
	At December 31, 2014, there were two series of Preferred Units outstanding (i.e., Series Four Preferred Units and Series B Preferred Units).
	•	The Series Four Preferred Units are not convertible into or exchangeable for any common equity of the Operating Partnership or the Company, have a per unit liquidation preference of $50.00 and are entitled to receive quarterly distributions of $0.25 per unit (or an annual rate of 2.00%) (See Note 11).
	•	The Series B Preferred Units were issued to the Company in connection with the Company's issuance of 80,000 shares (8,000,000 depositary shares each representing 1/100th of a share) of 5.25% Series B Cumulative Redeemable Preferred Stock (the "Series B Preferred Stock"). The Company contributed the net proceeds from the offering to the Operating Partnership in exchange for 80,000 Series B Preferred Units having terms and preferences generally mirroring those of the Series B Preferred Stock (See Note 12).
	All references herein to the Company refer to Boston Properties, Inc. and its consolidated subsidiaries, including the Operating Partnership, collectively, unless the context otherwise requires.
	Properties
	At December 31, 2014, the Company owned or had interests in a portfolio of 169 commercial real estate properties (the “Properties”) aggregating approximately 45.8 million net rentable square feet, including ten properties under construction totaling approximately 3.3 million net rentable square feet. In addition, the Company has structured parking for approximately 43,824 vehicles containing approximately 15.0 million square feet. At December 31, 2014, the Properties consist of:
	•	160 office properties, including 129 Class A office properties (including nine properties under construction) and 31 Office/Technical properties;
	•	one hotel;
	•	five retail properties (including one property under construction); and
	•	three residential properties.
	The Company owns or controls undeveloped land parcels totaling approximately 490.8 acres.
	The Company considers Class A office properties to be centrally located buildings that are professionally managed and maintained, attract high-quality tenants and command upper-tier rental rates, and that are modern structures or have been modernized to compete with newer buildings. The Company considers Office/Technical properties to be properties that support office, research and development, laboratory and other technical uses. The Company’s definitions of Class A Office and Office/Technical properties may be different than those used by other companies. Net rentable square feet amounts are unaudited.
	Basis of Presentation
	Boston Properties, Inc. does not have any other significant assets, liabilities or operations, other than its investment in the Operating Partnership, nor does it have employees of its own. The Operating Partnership, not Boston Properties, Inc., generally executes all significant business relationships other than transactions involving securities of Boston Properties, Inc. All majority-owned subsidiaries and joint ventures over which the Company has financial and operating control and variable interest entities ("VIE"s) in which the Company has determined it is the primary beneficiary are included in the consolidated financial statements. At December 31, 2014 and 2013, the Company did not have any VIEs. All significant intercompany balances and transactions have been eliminated in consolidation. The Company accounts for all other unconsolidated joint ventures using the equity method of accounting. Accordingly, the Company’s share of the earnings of these joint ventures and companies is included in consolidated net income.

Summary_Of_Significant_Account

Summary Of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2014
Accounting Policies [Abstract]
Basis Of Presentation And Summary Of Significant Accounting Policies	Summary of Significant Accounting Policies
	Real Estate
	Upon acquisitions of real estate that constitutes a business, which includes the consolidation of previously unconsolidated joint ventures, the Company assesses the fair value of acquired tangible and intangible assets (including land, buildings, tenant improvements, “above-” and “below-market” leases, leasing and assumed financing origination costs, acquired in-place leases, other identified intangible assets and assumed liabilities) and allocates the purchase price to the acquired assets and assumed liabilities, including land and buildings as if vacant. The Company assesses and considers fair value based on estimated cash flow projections that utilize discount and/or capitalization rates that it deems appropriate, as well as available market information. Estimates of future cash flows are based on a number of factors including the historical operating results, known and anticipated trends, and market and economic conditions.
	The fair value of the tangible assets of an acquired property considers the value of the property as if it were vacant. The Company also considers an allocation of purchase price of other acquired intangibles, including acquired in-place leases that may have a customer relationship intangible value, including (but not limited to) the nature and extent of the existing relationship with the tenants, the tenants' credit quality and expectations of lease renewals. Based on its acquisitions to date, the Company's allocation to customer relationship intangible assets has been immaterial.
	The Company records acquired “above-” and “below-market” leases at their fair values (using a discount rate which reflects the risks associated with the leases acquired) equal to the difference between (1) the contractual amounts to be paid pursuant to each in-place lease and (2) management’s estimate of fair market lease rates for each corresponding in-place lease, measured over a period equal to the remaining term of the lease for above-market leases and the initial term plus the term of any below-market fixed rate renewal options for below- market leases. Acquired “above-” and “below-market” lease values have been reflected within Prepaid Expenses and Other Assets and Other Liabilities, respectively, in the Company's Consolidated Balance Sheets. Other intangible assets acquired include amounts for in-place lease values that are based on the Company's evaluation of the specific characteristics of each tenant's lease. Factors to be considered include estimates of carrying costs during hypothetical expected lease-up periods considering current market conditions, and costs to execute similar leases. In estimating carrying costs, the Company includes real estate taxes, insurance and other operating expenses and estimates of lost rentals at market rates during the expected lease-up periods, depending on local market conditions. In estimating costs to execute similar leases, the Company considers leasing commissions, legal and other related expenses.
	Management reviews its long-lived assets for impairment following the end of each quarter and when there is an event or change in circumstances that indicates an impairment in value. An impairment loss is recognized if the carrying amount of its assets is not recoverable and exceeds its fair value. If such criteria are present, an impairment loss is recognized based on the excess of the carrying amount of the asset over its fair value. The evaluation of anticipated cash flows is highly subjective and is based in part on assumptions regarding future occupancy, rental rates and capital requirements that could differ materially from actual results in future periods. Since cash flows on properties considered to be “long-lived assets to be held and used” are considered on an undiscounted basis to determine whether an asset has been impaired, the Company's established strategy of holding properties over the long term directly decreases the likelihood of recording an impairment loss. If the Company's strategy changes or market conditions otherwise dictate an earlier sale date, an impairment loss may be recognized and such loss could be material. If the Company determines that an impairment has occurred, the affected assets must be reduced to their fair value, less cost to sell.
	Guidance in Accounting Standards Codification (“ASC”) 360 “Property Plant and Equipment” (“ASC 360”) requires that qualifying assets and liabilities and the results of operations that have been sold, or otherwise qualify as “held for sale,” be presented as discontinued operations in all periods presented if the property operations are expected to be eliminated and the Company will not have significant continuing involvement following the sale. The components of the property’s net income that are reflected as discontinued operations include the net gain (or loss) upon the disposition of the property held for sale, operating results, depreciation and interest expense (if the property is subject to a secured loan). The Company generally considers assets to be “held for sale” when the transaction has been approved by the Board of Directors, or a committee thereof, and there are no known significant contingencies relating to the sale, such that a sale of the property within one year is considered probable. Following the classification of a property as “held for sale,” no further depreciation is recorded on the assets, and the asset is written down to the lower of carrying value or fair market value, less cost to sell. On April 10, 2014, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) 2014-08, "Reporting Discontinued Operations and Disclosures of Disposals of Components of an Entity" (“ASU 2014-08”). ASU 2014-08 clarifies that discontinued operations presentation applies only to disposals representing a strategic shift that has (or will have) a major effect on an entity’s operations and financial results (e.g., a disposal of a major geographical area, a major line of business, a major equity method investment or other major parts of an entity). ASU 2014-08 is effective prospectively for reporting periods beginning after December 15, 2014. Early adoption is permitted, and the Company early adopted ASU 2014-08 during the first quarter of 2014. The Company’s adoption of ASU 2014-08 resulted in the operating results and gains on sales of real estate from operating properties sold during the year ended December 31, 2014 not being reflected within Discontinued Operations in the Company's Consolidated Statements of Operations (See Note 3).
	Real estate is stated at depreciated cost. A variety of costs are incurred in the acquisition, development and leasing of properties. The cost of buildings and improvements includes the purchase price of property, legal fees and other acquisition costs. The Company expenses costs that it incurs to effect a business combination such as legal, due diligence and other closing related costs. Costs directly related to the development of properties are capitalized. Capitalized development costs include interest, internal wages, property taxes, insurance, and other project costs incurred during the period of development. After the determination is made to capitalize a cost, it is allocated to the specific component of a project that is benefited. Determination of when a development project commences and capitalization begins, and when a development project is substantially complete and held available for occupancy and capitalization must cease, involves a degree of judgment. The Company’s capitalization policy on development properties is guided by guidance in ASC 835-20 “Capitalization of Interest” and ASC 970 “Real Estate-General.” The costs of land and buildings under development include specifically identifiable costs.
	The capitalized costs include pre-construction costs necessary to the development of the property, development costs, construction costs, interest costs, real estate taxes, salaries and related costs and other costs incurred during the period of development. The Company begins the capitalization of costs during the pre-construction period which it defines as activities that are necessary to the development of the property. The Company considers a construction project as substantially completed and held available for occupancy upon the completion of tenant improvements, but no later than one year from cessation of major construction activity. The Company ceases capitalization on the portion (1) substantially completed, (2) occupied or held available for occupancy, and capitalizes only those costs associated with the portion under construction or (3) if activities necessary for the development of the property have been suspended. Interest costs capitalized for the years ended December 31, 2014, 2013 and 2012 were $52.5 million, $68.2 million and $44.3 million, respectively. Salaries and related costs capitalized for the years ended December 31, 2014, 2013 and 2012 were $8.5 million, $7.7 million and $7.1 million, respectively.
	Expenditures for repairs and maintenance are charged to operations as incurred. Significant betterments are capitalized. When assets are sold or retired, their costs and related accumulated depreciation are removed from the accounts with the resulting gains or losses reflected in net income or loss for the period.
	The Company computes depreciation and amortization on properties using the straight-line method based on estimated useful asset lives. In accordance with ASC 805, the Company allocates the acquisition cost of real estate to its components and depreciates or amortizes these assets (or liabilities) over their useful lives. The amortization of acquired “above-” and “below-market” leases and acquired in-place leases is recorded as an adjustment to revenue and depreciation and amortization, respectively, in the Consolidated Statements of Operations.
	Depreciation is computed on a straight-line basis over the estimated useful lives of the assets as follows:
	Land improvements		25 to 40 years
	Buildings and improvements		10 to 40 years
	Tenant improvements		Shorter of useful life or terms of related lease
	Furniture, fixtures, and equipment		3 to 7 years
	Cash and Cash Equivalents
	Cash and cash equivalents consist of cash on hand and investments with maturities of three months or less from the date of purchase. The majority of the Company’s cash and cash equivalents are held at major commercial banks which may at times exceed the Federal Deposit Insurance Corporation limit of $250,000. The Company has not experienced any losses to date on its invested cash.
	Cash Held in Escrows
	Escrows include amounts established pursuant to various agreements for security deposits, property taxes, insurance and other costs. Escrows also include cash held by qualified intermediaries for possible investments in a like-kind exchanges in accordance with Section 1031 of the Internal Revenue Code in connection with sales of the Company’s properties.
	Investments in Securities
	The Company accounts for investments in trading securities at fair value, with gains or losses resulting from changes in fair value recognized currently in earnings. The designation of trading securities is generally determined at acquisition. The Company maintains a deferred compensation plan that is designed to allow officers of the Company to defer a portion of their current income on a pre-tax basis and receive a tax-deferred return on these deferrals. The Company’s obligation under the plan is that of an unsecured promise to pay the deferred compensation to the plan participants in the future. At December 31, 2014 and 2013, the Company had maintained approximately $19.5 million and $16.6 million, respectively, in a separate account, which is not restricted as to its use. The Company recognized gains of approximately $1.0 million, $2.9 million and $1.4 million on its investments in the account associated with the Company’s deferred compensation plan during the years ended December 31, 2014, 2013 and 2012, respectively.
	Tenant and Other Receivables
	Tenant and other accounts receivable, other than accrued rents receivable, are expected to be collected within one year.
	Deferred Charges
	Deferred charges include leasing costs and financing fees. Leasing costs include an allocation for acquired intangible in-place lease values and direct and incremental fees and costs incurred in the successful negotiation of leases, including brokerage, legal, internal leasing employee salaries and other costs which have been deferred and are being amortized on a straight-line basis over the terms of the respective leases. Internal leasing salaries and related costs capitalized for the years ended December 31, 2014, 2013 and 2012 were $6.0 million, $5.1 million and $5.6 million, respectively. External fees and costs incurred to obtain long-term financing have been deferred and are being amortized over the terms of the respective loans and are included within interest expense. Unamortized financing and leasing costs are charged to expense upon the early repayment or significant modification of the financing or upon the early termination of the lease, respectively. Fully amortized deferred charges are removed from the books upon the expiration of the lease or maturity of the debt.
	Investments in Unconsolidated Joint Ventures
	The Company consolidates variable interest entities (“VIEs”) in which it is considered to be the primary beneficiary. VIEs are entities in which the equity investors do not have sufficient equity at risk to finance their endeavors without additional financial support or that the holders of the equity investment at risk do not have a controlling financial interest. The primary beneficiary is defined by the entity having both of the following characteristics: (1) the power to direct the activities that, when taken together, most significantly impact the variable interest entity's performance, and (2) the obligation to absorb losses and right to receive the returns from the variable interest entity that would be significant to the variable interest entity. For ventures that are not VIEs the Company consolidates entities for which it has significant decision making control over the ventures' operations. The Company's judgment with respect to its level of influence or control of an entity involves the consideration of various factors including the form of the Company's ownership interest, its representation in the entity's governance, the size of its investment (including loans), estimates of future cash flows, its ability to participate in policy making decisions and the rights of the other investors to participate in the decision making process and to replace the Company as manager and/or liquidate the venture, if applicable. The Company's assessment of its influence or control over an entity affects the presentation of these investments in the Company's consolidated financial statements. In addition to evaluating control rights, the Company consolidates entities in which the outside partner has no substantive kick-out rights to remove the Company as the managing member.
	Accounts of the consolidated entity are included in the accounts of the Company and the non-controlling interest is reflected on the Consolidated Balance Sheets as a component of equity or in temporary equity between liabilities and equity. Investments in unconsolidated joint ventures are recorded initially at cost, and subsequently adjusted for equity in earnings and cash contributions and distributions. Any difference between the carrying amount of these investments on the balance sheet and the underlying equity in net assets is amortized as an adjustment to equity in earnings of unconsolidated joint ventures over the life of the related asset. Under the equity method of accounting, the net equity investment of the Company is reflected within the Consolidated Balance Sheets, and the Company's share of net income or loss from the joint ventures is included within the Consolidated Statements of Operations. The joint venture agreements may designate different percentage allocations among investors for profits and losses; however, the Company's recognition of joint venture income or loss generally follows the joint venture's distribution priorities, which may change upon the achievement of certain investment return thresholds. The Company may account for cash distributions in excess of its investment in an unconsolidated joint venture as income when the Company is not the general partner in a limited partnership and when the Company has neither the requirement nor the intent to provide financial support to the joint venture. The Company's investments in unconsolidated joint ventures are reviewed for impairment periodically and the Company records impairment charges when events or circumstances change indicating that a decline in the fair values below the carrying values has occurred and such decline is other-than-temporary. The ultimate realization of the investment in unconsolidated joint ventures is dependent on a number of factors, including the performance of each investment and market conditions. The Company will record an impairment charge if it determines that a decline in the value below the carrying value of an investment in an unconsolidated joint venture is other-than-temporary.
	To the extent that the Company contributes assets to a joint venture, the Company’s investment in the joint venture is recorded at the Company’s cost basis in the assets that were contributed to the joint venture. To the extent that the Company’s cost basis is different than the basis reflected at the joint venture level, the basis difference is amortized over the life of the related asset and included in the Company’s share of equity in net income of the joint venture. In accordance with the provisions of ASC 970-323 “Investments—Equity Method and Joint Ventures” (“ASC 970-323”), the Company will recognize gains on the contribution of real estate to joint ventures, relating solely to the outside partner’s interest, to the extent the economic substance of the transaction is a sale.
	The combined summarized financial information of the unconsolidated joint ventures is disclosed in Note 5 to the Consolidated Financial Statements.
	Equity Offering Costs
	Underwriting commissions and offering costs have been reflected as a reduction of additional paid-in capital.
	Treasury Stock
	The Company’s share repurchases are reflected as treasury stock utilizing the cost method of accounting and are presented as a reduction to consolidated stockholders’ equity.
	Dividends
	Earnings and profits, which determine the taxability of dividends to stockholders, will differ from income reported for financial reporting purposes due to the differences for federal income tax purposes in the treatment of gains/losses on the sale of real property, revenue and expense recognition, compensation expense, and in the estimated useful lives and basis used to compute depreciation.
	The tax treatment of common dividends per share for federal income tax purposes is as follows:
			For the year ended December 31,
			2014							2013								2012
			Per Share				%			Per Share				%				Per Share			%
	Ordinary income		$	—			—	%		$	2.31			48.71	%	0.9093		$	2.29	0.9093	96.45	%
	Capital gain income		6.82				100	%		2.44				51.29	%	—		0.08		—	3.55	%
	Total		$	6.82		-1	100	%		$	4.75		-2	100	%			$	2.37	1	100	%
	_____________
	(1) The fourth quarter 2014 dividend of $5.15 per common share consists of a $4.50 per common share special dividend and a $0.65 per common share regular quarterly dividend, approximately $4.41 per common share was allocable to 2014 and approximately $0.74 per common share is allocable to 2015.
	(2) The fourth quarter 2013 dividend of $2.90 per common share consists of a $2.25 per common share special dividend and a $0.65 per common share regular quarterly dividend, approximately $2.44 per common share was allocable to 2013 and approximately $0.46 per common share is allocable to 2014.
	Revenue Recognition
	In general, the Company commences rental revenue recognition when the tenant takes possession of the leased space and the leased space is substantially ready for its intended use. Contractual rental revenue is reported on a straight-line basis over the terms of the respective leases. The impact of the straight-line rent adjustment increased revenue by approximately $63.1 million, $65.8 million and $77.6 million for the years ended December 31, 2014, 2013 and 2012, respectively, as the revenue recorded exceeded amounts billed. Accrued rental income, as reported on the Consolidated Balance Sheets, represents cumulative rental income earned in excess of rent payments received pursuant to the terms of the individual lease agreements. The Company maintains an allowance against accrued rental income for future potential tenant credit losses. The credit assessment is based on the estimated accrued rental income that is recoverable over the term of the lease. The Company also maintains an allowance for doubtful accounts for estimated losses resulting from the inability of tenants to make required rent payments. The computation of this allowance is based on the tenants’ payment history and current credit status, as well as certain industry or geographic specific credit considerations. If the Company’s estimates of collectability differ from the cash received, then the timing and amount of the Company’s reported revenue could be impacted. The credit risk is mitigated by the high quality of the Company’s existing tenant base, reviews of prospective tenants’ risk profiles prior to lease execution and consistent monitoring of the Company’s portfolio to identify potential problem tenants.
	In accordance with ASC 805, the Company recognizes rental revenue of acquired in-place “above-” and “below-market” leases at their fair values over the original term of the respective leases. The impact of the acquired in-place “above-” and “below-market” leases increased revenue by approximately $48.3 million, $28.0 million and $14.6 million for the years ended December 31, 2014, 2013 and 2012, respectively. The following table summarizes the scheduled amortization of the Company's acquired “above-” and “below-market” lease intangibles for each of the five succeeding years (in thousands). Accrued rental income as reported on the Consolidated Balance Sheets represents rental income recognized in excess of rent payments actually received pursuant to the terms of the individual lease agreements.
			Acquired Above-Market Lease Intangibles				Acquired Below-Market Lease Intangibles
	2015		$	22,671			$	57,019
	2016		20,491				51,460
	2017		12,277				35,896
	2018		8,637				33,215
	2019		7,106				27,615
	Recoveries from tenants, consisting of amounts due from tenants for common area maintenance, real estate taxes and other recoverable costs, are recognized as revenue in the period during which the expenses are incurred. Tenant reimbursements are recognized and presented in accordance with guidance in ASC 605-45 “Principal Agent Considerations” (“ASC 605-45”). ASC 605-45 requires that these reimbursements be recorded on a gross basis, as the Company is generally the primary obligor with respect to purchasing goods and services from third-party suppliers, has discretion in selecting the supplier and has credit risk. The Company also receives reimbursement of payroll and payroll related costs from third parties which the Company reflects on a net basis.
	The Company's parking revenues are derived from leases, monthly parking and transient parking. The Company recognizes parking revenue as earned.
	The Company’s hotel revenues are derived from room rentals and other sources such as charges to guests for telephone service, movie and vending commissions, meeting and banquet room revenue and laundry services. Hotel revenues are recognized as earned.
	The Company receives management and development fees from third parties. Property management fees are recorded and earned based on a percentage of collected rents at the properties under management, and not on a straight-line basis, because such fees are contingent upon the collection of rents. The Company records development fees as earned depending on the risk associated with each project. Profit on development fees earned from joint venture projects is recognized as revenue to the extent of the third party partners’ ownership interest.
	Gains on sales of real estate are recognized pursuant to the provisions included in ASC 360-20 “Real Estate Sales” (“ASC 360-20”). The specific timing of the sale is measured against various criteria in ASC 360-20 related to the terms of the transaction and any continuing involvement in the form of management or financial assistance associated with the properties. If the sales criteria for the full accrual method are not met, the Company defers some or all of the gain recognition and accounts for the continued operations of the property by applying the finance, leasing, profit sharing, deposit, installment or cost recovery methods, as appropriate, until the sales criteria are met.
	Depreciation and Amortization
	The Company computes depreciation and amortization on its properties using the straight-line method based on estimated useful asset lives. The Company allocates the acquisition costs of real estate to its components and depreciate or amortize these assets over their useful lives. The amortization of acquired “above-” and “below-market” leases and acquired in-place leases is recorded as an adjustment to revenue and depreciation and amortization, respectively, in the Consolidated Statements of Operations.
	Ground Leases
The Company has non-cancelable ground lease obligations with various initial term expiration dates through 2068 (See Note 20). The Company recognizes ground rent expense on a straight-line basis over the terms of the respective ground lease agreements. The future contractual minimum lease payments to be made by the Company as of December 31, 2014, under non-cancelable ground leases which expire on various dates through 2068, are as follows:
Years Ending December 31,	(in thousands)
2015	$	13,507
2016	13,732
2017	13,963
2018	14,198
2019	14,461
Thereafter	884,726
Earnings Per Share
Basic earnings per share (“EPS”) is computed by dividing net income available to common shareholders, as adjusted for undistributed earnings (if any) of certain securities issued by the Operating Partnership, by the weighted average number of shares of Common Stock outstanding during the year. Diluted EPS reflects the potential dilution that could occur from shares issuable in connection with awards under stock-based compensation plans, including upon the exercise of stock options, and securities of the Operating Partnership that are exchangeable for Common Stock.
Fair Value of Financial Instruments
The carrying values of cash and cash equivalents, marketable securities, escrows, receivables, accounts payable, accrued expenses and other assets and liabilities are reasonable estimates of their fair values because of the short maturities of these instruments.
The Company follows the authoritative guidance for fair value measurements when valuing its financial instruments for disclosure purposes. The Company determines the fair value of its unsecured senior notes and unsecured exchangeable senior notes using market prices. The inputs used in determining the fair value of the Company’s unsecured senior notes and unsecured exchangeable senior notes is categorized at a level 1 basis (as defined in the accounting standards for Fair Value Measurements and Disclosures) due to the fact that the Company uses quoted market rates to value these instruments. However, the inputs used in determining the fair value could be categorized at a level 2 basis if trading volumes are low. The Company determines the fair value of its mortgage notes payable using discounted cash flow analyses by discounting the spread between the future contractual interest payments and hypothetical future interest payments on mortgage debt based on current market rates for similar securities. In determining the current market rates, the Company adds its estimates of market spreads to the quoted yields on federal government treasury securities with similar maturity dates to its debt. The inputs used in determining the fair value of the Company’s mortgage notes payable and mezzanine notes payable are categorized at a level 3 basis (as defined in the accounting standards for Fair Value Measurements and Disclosures) due to the fact that the Company considers the rates used in the valuation techniques to be unobservable inputs.
Because the Company’s valuations of its financial instruments are based on these types of estimates, the actual fair values of its financial instruments may differ materially if the Company’s estimates do not prove to be accurate. The following table presents the aggregate carrying value of the Company’s indebtedness and the Company’s corresponding estimate of fair value as of December 31, 2014 and December 31, 2013 (in thousands):
	December 31, 2014								December 31, 2013
	Carrying				Estimated				Carrying						Estimated
	Amount				Fair Value				Amount						Fair Value
Mortgage notes payable	$	4,309,484			$	4,449,541			$	4,449,734					$	4,545,283
Mezzanine notes payable	309,796				306,156				311,040						311,064
Unsecured senior notes	5,287,704				5,645,819				5,835,854						6,050,517
Unsecured exchangeable senior notes	—				—				744,880				-1		750,266
Total	$	9,906,984			$	10,401,516			$	11,341,508					$	11,657,130
_______________
-1	Includes the net adjustment for the equity component allocation totaling approximately $2.4 million at December 31, 2013.
Derivative Instruments and Hedging Activities
Derivative instruments and hedging activities require management to make judgments on the nature of its derivatives and their effectiveness as hedges. These judgments determine if the changes in fair value of the derivative instruments are reported in the Consolidated Statements of Operations as a component of net income or as a component of comprehensive income and as a component of equity on the Consolidated Balance Sheets. While management believes its judgments are reasonable, a change in a derivative’s effectiveness as a hedge could materially affect expenses, net income and equity. The Company accounts for the effective portion of changes in the fair value of a derivative in other comprehensive income (loss) and subsequently reclassifies the effective portion to earnings over the term that the hedged transaction affects earnings. The Company accounts for the ineffective portion of changes in the fair value of a derivative directly in earnings.
Income Taxes
The Company has elected to be treated as a REIT under Sections 856 through 860 of the Internal Revenue Code of 1986, as amended (the “Code”), commencing with its taxable year ended December 31, 1997. As a result, the Company generally will not be subject to federal corporate income tax on its taxable income that is distributed to its stockholders. A REIT is subject to a number of organizational and operational requirements, including a requirement that it currently distribute at least 90% of its annual taxable income (with certain adjustments). The Company’s policy is to distribute at least 100% of its taxable income. Accordingly, the only provision for federal income taxes in the accompanying consolidated financial statements relates to the Company’s consolidated taxable REIT subsidiaries. The Company’s taxable REIT subsidiaries did not have significant tax provisions or deferred income tax items. The Company has no uncertain tax positions recognized as of December 31, 2014 and 2013.
The Company owns a hotel property which is managed through a taxable REIT subsidiary. The hotel taxable REIT subsidiary, a wholly owned subsidiary of the Operating Partnership, is the lessee pursuant to the lease for the hotel property. As lessor, the Operating Partnership is entitled to a percentage of gross receipts from the hotel property. Marriott International, Inc. continues to manage the hotel property under the Marriott name and under terms of the existing management agreement. The hotel taxable REIT subsidiary is subject to tax at the federal and state level and, accordingly, the Company has recorded a tax provision in the Company’s Consolidated Statements of Operations for the years ended December 31, 2014, 2013 and 2012.
The net difference between the tax basis and the reported amounts of the Company’s assets and liabilities is approximately $2.2 billion and $1.7 billion as of December 31, 2014 and 2013, respectively, which is primarily related to the difference in basis of contributed property and accrued rental income.
Certain entities included in the Company’s consolidated financial statements are subject to certain state and local taxes. These taxes are recorded as operating expenses in the accompanying consolidated financial statements.
The following table reconciles GAAP net income attributable to Boston Properties, Inc. to taxable income:
		For the year ended December 31,
		2014				2013				2012
		(in thousands)
Net income attributable to Boston Properties, Inc.		$	443,611			$	749,811			$	289,650
Straight-line rent adjustments		(91,733		)		(74,445		)		(80,281		)
Book/Tax differences from depreciation and amortization		239,681				170,370				105,599
Book/Tax differences from interest expense		(43,148		)		(7,912		)		27,660
Book/Tax differences on gains/losses from capital transactions		943,778				(124,413		)		(22,408		)
Book/Tax differences from stock-based compensation		32,483				42,146				19,660
Tangible Property Regulations (1)		(442,650		)		—				—
Other book/tax differences, net		(7,945		)		(4,885		)		7,801
Taxable income		$	1,074,077			$	750,672			$	347,681
__________
(1) In September 2013, the Internal Revenue Service released final Regulations governing when taxpayers like the Company must capitalize and depreciate costs for acquiring, maintaining, repairing and replacing tangible property and when taxpayers can deduct such costs.  These final Regulations are effective for tax years beginning on or after January 1, 2014.  These Regulations permitted the Company to deduct certain types of expenditures that were previously required to be capitalized.  The Regulations also allowed the Company to make a one-time election to immediately deduct certain amounts that were capitalized in previous years that are not required to be capitalized under the new Regulations. The one-time deduction included above totaled approximately $385.6 million.
Stock-Based Employee Compensation Plans
At December 31, 2014, the Company has a stock-based employee compensation plan. Effective January 1, 2005, the Company adopted early ASC 718 “Compensation – Stock Compensation” (“ASC 718”), which revised the fair value based method of accounting for share-based payment liabilities, forfeitures and modifications of stock-based awards and clarified previous guidance in several areas, including measuring fair value, classifying an award as equity or as a liability and attributing compensation cost to reporting periods.
Use of Estimates in the Preparation of Financial Statements
The preparation of financial statements in conformity with accounting principles generally accepted in the United States of America requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenue and expenses during the reporting period. These estimates include such items as depreciation and allowances for doubtful accounts. Actual results could differ from those estimates.
Out-of-Period Adjustment
During the year ended December 31, 2014, the Company recorded an allocation of net income to the noncontrolling interest holder in its Fountain Square consolidated joint venture totaling approximately $1.9 million related to the cumulative non-cash adjustment to the accretion of the changes in the redemption value of the noncontrolling interest. This resulted in the overstatement of Noncontrolling Interests in Property Partnerships by approximately $1.9 million during the year ended December 31, 2014 and an understatement of Noncontrolling Interests in Property Partnerships in the aggregate amount of approximately $1.9 million in periods prior to 2014. Because this adjustment was not material to the prior periods’ consolidated financial statements and the impact of recording the adjustment in 2014 was not material to the Company’s consolidated financial statements, the Company recorded the related adjustment during the year ended December 31, 2014. The out of period adjustment was identified and recorded during the second quarter of 2014.
Revision
The Company revised the presentation of certain investments in unconsolidated joint ventures with deficit balances to reflect the deficit balances within Other Liabilities on the Company's Consolidated Balance Sheets instead of within Investments in Unconsolidated Joint Ventures. The revision resulted in an aggregate of approximately $14.0 million at December 31, 2013 being presented within Other Liabilities on the Company's Consolidated Balance Sheets, which revision was not material to the period.
Recent Accounting Pronouncements
On April 10, 2014, the FASB issued ASU 2014-08, "Reporting Discontinued Operations and Disclosures of Disposals of Components of an Entity" (“ASU 2014-08”). ASU 2014-08 clarifies that discontinued operations presentation applies only to disposals representing a strategic shift that has (or will have) a major effect on an entity’s operations and financial results (e.g., a disposal of a major geographical area, a major line of business, a major equity method investment or other major parts of an entity). ASU 2014-08 is effective prospectively for reporting periods beginning after December 15, 2014. Early adoption is permitted, and the Company early adopted ASU 2014-08 during the first quarter of 2014. The Company’s adoption of ASU 2014-08 resulted in the operating results and gains on sales of real estate from operating properties sold during the year ended December 31, 2014 not being reflected within Discontinued Operations in the Company's Consolidated Statements of Operations (See Note 3).
In May 2014, the FASB issued ASU 2014-09, "Revenue from Contracts with Customers (Topic 606)" ("ASU 2014-09"). The objective of ASU 2014-09 is to establish a single comprehensive model for entities to use in accounting for revenue arising from contracts with customers and will supersede most of the existing revenue recognition guidance, including industry-specific guidance. The core principle is that an entity should recognize revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. In applying ASU 2014-09, companies will perform a five-step analysis of transactions to determine when and how revenue is recognized. ASU 2014-09 applies to all contracts with customers except those that are within the scope of other topics in the FASB's Accounting Standards Codification ("ASC"). ASU 2014-09 is effective for annual reporting periods (including interim periods within that reporting period) beginning after December 15, 2016 and shall be applied using either a full retrospective or modified retrospective approach. Early adoption is not permitted. The Company is currently assessing the potential impact that the adoption of ASU 2014-09 will have on its consolidated financial statements.
In June 2014, the FASB issued ASU 2014-12, “Accounting for Share-Based Payments When the Terms of an Award Provide That a Performance Target Could Be Achieved after the Requisite Service Period” (“ASU 2014-12”). The amendments in ASU 2014-12 require that a performance target that affects vesting and that could be achieved after the requisite service period be treated as a performance condition. A reporting entity should apply existing guidance in ASC Topic No. 718, “Compensation - Stock Compensation” (“ASC 718”), as it relates to awards with performance conditions that affect vesting to account for such awards. The amendments in ASU 2014-12 are effective for annual periods and interim periods within those annual periods beginning after December 15, 2015. Early adoption is permitted. Entities may apply the amendments in ASU 2014-12 either: (a) prospectively to all awards granted or modified after the effective date; or (b) retrospectively to all awards with performance targets that are outstanding as of the beginning of the earliest annual period presented in the financial statements and to all new or modified awards thereafter. The Company does not expect the adoption of ASU 2014-12 to have a material impact on its consolidated financial statements.
In August 2014, the FASB issued ASU 2014-15, "Presentation of Financial Statements - Going Concern: Disclosure of Uncertainties about an Entity’s Ability to Continue as a Going Concern" (“ASU 2014-15”). ASU 2014-15 requires an entity to evaluate whether there are conditions or events that raise substantial doubt about the entity’s ability to continue as a going concern within one year after the date that the financial statements are issued (or within one year after the financial statements are available to be issued when applicable) and to provide related footnote disclosures in certain circumstances. ASU 2014-15 is effective for the annual period ending after December 15, 2016, and for annual and interim periods thereafter with early adoption permitted. The Company does not expect the adoption of ASU 2014-15 to have a material impact on its consolidated financial statements.
In November 2014, the FASB issued ASU 2014-16, "Determining Whether the Host Contract in a Hybrid Financial Instrument Issued in the Form of a Share Is More Akin to Debt or to Equity" (“ASU 2014-16”). ASU 2014-16 clarifies how current GAAP should be interpreted in evaluating the economic characteristics and risks of a host contract in a hybrid financial instrument that is issued in the form of a share. Specifically, the amendments clarify that an entity should consider all relevant terms and features—including the embedded derivative feature being evaluated for bifurcation—in evaluating the nature of the host contract. Furthermore, the amendments clarify that no single term or feature would necessarily determine the economic characteristics and risks of the host contract. Rather, the nature of the host contract depends upon the economic characteristics and risks of the entire hybrid financial instrument. ASU 2014-16 is effective for fiscal years and interim periods beginning after December 15, 2015. Early adoption is permitted. The Company does not expect the adoption of ASU 2014-16 to have a material impact on its consolidated financial statements.
In February 2015, the FASB issued ASU 2015-02, “Consolidation (Topic 810): Amendments to the Consolidation Analysis” (“ASU 2015-02”). ASU 2015-02 affects reporting entities that are required to evaluate whether they should consolidate certain legal entities. ASU 2015-02 modifies the evaluation of whether limited partnerships and similar legal entities are VIEs or voting interest entities, eliminates the presumption that a general partner should consolidate a limited partnership and affects the consolidation analysis of reporting entities that are involved with VIEs, particularly those that have fee arrangements and related party relationships. ASU 2015-02 is effective for fiscal years, and for interim periods within those fiscal years, beginning after December 15, 2015. Early adoption is permitted. A reporting entity may apply the amendments in ASU 2015-02 using: (a) a modified retrospective approach by recording a cumulative-effect adjustment to equity as of the beginning of the fiscal year of adoption; or (b) by applying the amendments retrospectively. The Company is currently assessing the potential impact that the adoption of ASU 2015-02 will have on its consolidated financial statements.portion

Real_Estate

Real Estate	12 Months Ended
	Dec. 31, 2014
Real Estate [Abstract]
Real Estate	Real Estate
	Real estate consisted of the following at December 31 (in thousands):
			2014				2013
	Land		$	4,785,772			$	4,450,532
	Land held for future development		268,114				297,376
	Buildings and improvements		11,666,105				11,065,113
	Tenant improvements		1,752,115				1,617,401
	Furniture, fixtures and equipment		27,986				25,164
	Construction in progress		736,311				1,523,179
	Total		19,236,403				18,978,765
	Less: Accumulated depreciation		(3,547,659		)		(3,161,571		)
			$	15,688,744			$	15,817,194
	Developments
	On February 10, 2014, the Company completed and fully placed in-service The Avant at Reston Town Center development project comprised of 359 apartment units and retail space aggregating approximately 355,000 square feet located in Reston, Virginia.
	On April 1, 2014, the Company commenced construction of its 99 Third Avenue development project totaling approximately 17,000 net rentable square feet of retail space located in Waltham, Massachusetts.
	On April 3, 2014, the Company commenced construction of its 690 Folsom Street development project totaling approximately 25,000 net rentable square feet of office and retail space located in San Francisco, California. This project was partially placed in-service on December 2, 2014.
	On April 10, 2014, a consolidated joint venture in which the Company has a 95% interest signed a lease with salesforce.com for 714,000 square feet at the new Salesforce Tower, the 1.4 million square foot, 61-story Class A office development project currently under construction at 415 Mission Street in the South Financial District of San Francisco, California. In conjunction with the lease signing, the Company has commenced construction of the building.
	On May 20, 2014, the Company commenced construction of its 888 Boylston Street development project totaling approximately 425,000 net rentable square feet of Class A office space located in Boston, Massachusetts.
	On May 20, 2014, the Company commenced construction of its 10 CityPoint development project totaling approximately 245,000 net rentable square feet of Class A office space located in Waltham, Massachusetts.
	On August 31, 2014, the Company completed and fully placed in-service 250 West 55th Street, a Class A office project with approximately 988,000 net rentable square feet located in New York City.
	On September 17, 2014, the Company completed and fully placed in-service 680 Folsom Street, a Class A office project with approximately 525,000 net rentable square feet located in San Francisco, California.
	On November 1, 2014, the Company partially placed in-service 535 Mission Street, a Class A office project with approximately 307,000 net rentable square feet located in San Francisco, California.
	Dispositions
	On July 29, 2014, the Company completed the sale of its Mountain View Technology Park properties and Mountain View Research Park Building Sixteen property located in Mountain View, California for an aggregate sale price of approximately $92.1 million. Net cash proceeds totaled approximately $90.6 million, resulting in a gain on sale of real estate totaling approximately $35.9 million. Mountain View Technology Park is a seven-building complex of Office/Technical properties aggregating approximately 135,000 net rentable square feet. Mountain View Research Park Building Sixteen is an Office/Technical property with approximately 63,000 net rentable square feet.
	On August 20, 2014, a portion of the land parcel at the Company’s One Reston Overlook property located in Reston, Virginia was taken by eminent domain. Net cash proceeds totaled approximately $2.6 million, resulting in a gain on sale of real estate totaling approximately $1.2 million.
	On August 22, 2014, the Company completed the sale of a parcel of land within its Broad Run Business Park property located in Loudoun County, Virginia for a sale price of approximately $9.8 million. Net cash proceeds totaled approximately $9.7 million, resulting in a gain on sale of real estate totaling approximately $4.3 million. The parcel is an approximately 15.5 acre land parcel subject to a ground lease that was scheduled to expire on October 31, 2048 with a tenant that exercised its purchase option under the ground lease.
	On October 2, 2014, the Company completed the sale of its Patriots Park properties located in Reston, Virginia for a gross sale price of $321.0 million. Patriots Park consists of three Class A office properties aggregating approximately 706,000 net rentable square feet. Net cash proceeds totaled approximately $319.1 million, resulting in a gain on sale of real estate totaling approximately $84.6 million. The Company has agreed to provide rent support payments to the buyer with a maximum obligation of up to approximately $12.3 million related to the leasing of 17,762 net rentable square feet at the properties, which has been recorded as a reduction to the gain on sale. Patriots Park contributed approximately $8.2 million, $10.8 million and $5.3 million of net income to the Company for the period from January 1, 2014 through October 1, 2014 and the years ended December 31, 2013 and 2012, respectively.
	On October 22, 2014, the tenant exercised its right to purchase the Company’s 415 Main Street property (formerly Seven Cambridge Center) located in Cambridge, Massachusetts on February 1, 2016.  As part of its lease signed on July 14, 2004, the tenant was granted an option to purchase the building at the beginning of the 11th lease year for approximately $106 million. 415 Main Street is an Office/Technical property with approximately 231,000 net rentable square feet.
	On October 24, 2014, the Company completed the sale of a parcel of land at 130 Third Avenue in Waltham, Massachusetts that is permitted for 129,000 square feet for a sale price of approximately $14.3 million. Net cash proceeds totaled approximately $13.6 million, resulting in a gain on sale of real estate totaling approximately $8.3 million.
	On October 30, 2014, the Company completed the sale of a 45% interest in each of 601 Lexington Avenue in New York City and Atlantic Wharf Office Building and 100 Federal Street in Boston for an aggregate gross sale price of approximately $1.827 billion in cash, less the partner’s pro rata share of the indebtedness collateralized by 601 Lexington Avenue. Net cash proceeds totaled approximately $1.497 billion, after the payment of transaction costs. In connection with the sale, the Company formed a limited liability company for each property with the buyer and will provide customary property management and leasing services to the joint ventures. 601 Lexington Avenue is a 1,669,000 square foot Class A office complex located in Midtown Manhattan. The property consists of a 59-story tower as well as a six-story low-rise office and retail building. The property is subject to existing mortgage indebtedness of approximately $712.9 million. The Atlantic Wharf Office Building is a 791,000 square foot Class A office tower located on Boston's Waterfront. 100 Federal Street is a 1,323,000 square foot Class A office tower located in Boston's Financial District. The transaction did not qualify as a sale of real estate for financial reporting purposes as the Company continues to effectively control these properties and thus will continue to account for the properties on a consolidated basis in its financial statements. The Company has accounted for the transaction as an equity transaction and has recognized noncontrolling interest in its consolidated balance sheets totaling approximately $849.0 million, which is equal to 45% of the aggregate carrying value of the total equity of the properties immediately prior to the transaction. The difference between the net cash proceeds received and the noncontrolling interest recognized, which was approximately $648.4 million, has not been reflected as a gain on sale of real estate in the Company's consolidated statements of operations and has instead been reflected as an increase in Additional Paid-in Capital in the Company's Consolidated Balance Sheets.
	On December 30, 2014, the Company completed the conveyance to an unrelated third party of a condominium interest in its 75 Ames Street property located in Cambridge, Massachusetts. On May 23, 2011, the Company had entered into a ground lease for the vacant land parcel at 75 Ames Street and had also entered into a development agreement to serve as project manager for a 250,000 square foot research laboratory building to be developed on the site at the ground lessee’s expense and to also serve, upon completion of development, as property manager. Gross proceeds to the Company were approximately $56.8 million, including $11.4 million in development fees for the Company’s services, and were received beginning in May 2011. The cash received under the ground lease was initially recognized as unearned revenue and recognized over the 99-year term of the ground lease as ground lease revenue totaling approximately $459,000 per year prior to the conveyance of the condominium interest. The terms of the ground lease required the Company to form a condominium for the site upon completion of the development, at which time each party would subject their respective interests in the buildings and land to the condominium and would in turn be conveyed a condominium unit comprised of their respective building as well as an undivided ownership interest in the land. As a result of the conveyance and the transfer of title, the Company recognized a gain on sale of real estate totaling approximately $33.8 million.
	The Company did not have any dispositions during the year ended December 31, 2014 that qualified for discontinued operations presentation subsequent to its adoption of ASU 2014-08. The following table summarizes the income from discontinued operations related to One Preserve Parkway, 10 & 20 Burlington Mall Road, 1301 New York Avenue, 303 Almaden Boulevard, Montvale Center and Bedford Business Park and the related gains on sales of real estate, gain on forgiveness of debt and impairment loss for the years ended December 31, 2013 and 2012:
		For the year ended December 31,
		2013				2012
		(in thousands)
	Total revenue	$	20,138			$	32,607
	Expenses
	Operating	6,996				12,038
	Depreciation and amortization	4,760				8,169
	Total expenses	11,756				20,207
	Operating income	8,382				12,400
	Other expense
	Interest expense	360				2,594
	Income from discontinued operations	$	8,022			$	9,806
	Noncontrolling interest in income from discontinued operations – common units of the Operating Partnership	(803		)		(1,031		)
	Income from discontinued operations attributable to Boston Properties, Inc.	$	7,219			$	8,775
	Gains on sales of real estate from discontinued operations	$	112,829			$	36,877
	Gain on forgiveness of debt from discontinued operations	20,182				—
	Impairment loss from discontinued operations	(3,241		)		—
	Noncontrolling interest in gains on sales of real estate, gain on forgiveness of debt and impairment loss from discontinued operations – common units of the Operating Partnership	(13,348		)		(4,044		)
	Gains on sales of real estate, gain on forgiveness of debt and impairment loss from discontinued operations attributable to Boston Properties, Inc.	$	116,422			$	32,833
	Acquisitions
	On November 6, 2014, the Company entered into an option agreement pursuant to which the Company has been granted an option to purchase real property located at 425 Fourth Street in San Francisco, California. In connection with the execution of the agreement, the Company paid a non-refundable option payment to the current owner of $1.0 million.
	On November 12, 2014, the Company completed the acquisition of a parcel of land at 804 Carnegie Center in Princeton, New Jersey for a purchase price of approximately $3.7 million. 804 Carnegie Center is a build-to-suit project with approximately 130,000 net rentable square feet of Class A office space, which is currently under construction.
	Prior Year Acquisitions Included in Pro Forma Information
	The accompanying unaudited pro forma information for the years ended December 31, 2013 and 2012 is presented as if the operating property acquisitions of (1) Mountain View Research Park and Mountain View Technology Park on April 10, 2013 and the approximately $26.5 million gain on consolidation and (2) 767 Fifth Avenue (the General Motors Building) on May 31, 2013 and the approximately $359.5 million gain on consolidation, had occurred on January 1, 2012. This unaudited pro forma information is based upon the historical consolidated financial statements of the Company and should be read in conjunction with the consolidated financial statements and notes thereto. This pro forma information does not purport to represent what the actual results of operations of the Company would have been had the above occurred, nor do they purport to predict the results of operations of future periods. Additional information for these transactions are provided below.
	Pro Forma (Unaudited)	Year ended December 31,
	(in thousands, except per share data)	2013				2012
	Total revenue	$	2,257,098			$	2,149,391
	Income from continuing operations	$	302,354			$	634,457
	Net income attributable to Boston Properties, Inc.	$	400,017			$	626,174
	Basic earnings per share:
	Net income per share attributable to Boston Properties, Inc.	$	2.58			$	4.15
	Diluted earnings per share:
Net income per share attributable to Boston Properties, Inc.	$	2.57			$	4.13
On April 10, 2013, the Company acquired the Mountain View Research Park and Mountain View Technology Park properties from Boston Properties Office Value-Added Fund, L.P. (the "Value-Added Fund") for an aggregate net purchase price of approximately $233.1 million. Mountain View Research Park is a 16-building complex of Office/Technical properties aggregating approximately 604,000 net rentable square feet. Mountain View Technology Park is a seven-building complex of Office/Technical properties aggregating approximately 135,000 net rentable square feet. The following table summarizes the allocation of the aggregate purchase price of Mountain View Research Park and Mountain View Technology Park at the date of acquisition (in thousands) in accordance with the guidance in ASC 805 "Business Combinations."
Land	$	126,521
Building and improvements	82,451
Tenant improvements	7,326
In-place lease intangibles	23,279
Above-market rents	843
Below-market rents	(7,336		)
Net assets acquired	$	233,084
On May 31, 2013, the Company's two joint venture partners in 767 Venture, LLC (the entity that owns 767 Fifth Avenue (the General Motors Building) located in New York City) transferred all of their interests in the joint venture to third parties. 767 Fifth Avenue (the General Motors Building) is a Class A office property totaling approximately 1.8 million net rentable square feet. In connection with the transfer, the Company and its new joint venture partners modified the Company's relative decision making authority and consent rights with respect to the joint venture's assets and operations. These changes resulted in the Company having sufficient financial and operating control over 767 Venture, LLC such that, effective as of May 31, 2013, the Company accounts for the assets, liabilities and operations of 767 Venture, LLC on a consolidated basis in its financial statements instead of under the equity method of accounting (See Note 11). The following table summarizes the allocation of the aggregate purchase price of 767 Fifth Avenue (the General Motors Building) at the date of consolidation on May 31, 2013 (in thousands) in accordance with the guidance in ASC 805 "Business Combinations."
Real estate and related intangibles recorded upon consolidation
Land	$	1,796,252
Building and improvements	1,447,446
Tenant improvements	85,208
In-place lease intangibles	357,781
Above market rents	101,897
Below market rents	(239,641		)
Above market assumed debt adjustments	(192,943		)
	$	3,356,000
Debt recorded upon consolidation
Mortgage notes payable	$	(1,300,000	)
Mezzanine notes payable	(306,000		)
Members' notes payable	(450,000		)	-1
	$	(2,056,000	)
Working capital recorded upon consolidation
Cash and cash equivalents	$	79,468
Cash held in escrows	2,403
Tenant and other receivables	7,104
Prepaid expenses and other assets	4,269
Accounts payable and accrued expenses	(2,418		)
Accrued interest payable	(182,369		)	-2
Other liabilities	(6,304		)
	$	(97,847	)
Noncontrolling interest recorded upon consolidation
Noncontrolling interests	$	(520,000	)
Noncontrolling interests - working capital	39,139
	$	(480,861	)
Net assets recorded upon consolidation	$	721,292
_______________
-1	The Company's member loan totaling $270.0 million eliminates in consolidation.
-2	The Company's share of the accrued interest payable on the members' loans totaling approximately $105.5 million eliminates in consolidation.
Mountain View Research Park and Mountain View Technology Park contributed approximately $16.7 million of revenue and approximately $0.4 million of earnings to the Company for the period from April 10, 2013 through December 31, 2013. 767 Fifth Avenue (the General Motors Building) contributed approximately $168.4 million of revenue and approximately $8.4 million of earnings to the Company for the period from May 31, 2013 through December 31, 2013.

Deferred_Charges

Deferred Charges	12 Months Ended
	Dec. 31, 2014
Deferred Charges [Abstract]
Deferred Charges	4.     Deferred Charges
	Deferred charges consisted of the following at December 31, (in thousands):
		2014			2013
	Leasing costs, including lease related intangibles	$	1,234,192		$	1,183,204
	Financing costs	69,127			76,798
		1,303,319			1,260,002
	Less: Accumulated amortization	(471,575		)	(375,552		)
		$	831,744		$	884,450
	The following table summarizes the scheduled amortization of the Company's acquired in-place lease intangibles for each of the five succeeding years (in thousands).
		Acquired In-Place Lease Intangibles
	2015	$	66,390
	2016	55,327
	2017	38,812
	2018	33,964
	2019	27,439

Investments_in_Unconsolidated_

Investments in Unconsolidated Joint Ventures	12 Months Ended
	Dec. 31, 2014
Investments In Unconsolidated Joint Ventures [Abstract]
Investments In Unconsolidated Joint Ventures	Investments in Unconsolidated Joint Ventures
	The investments in unconsolidated joint ventures consist of the following at December 31, 2014:
	Entity		Properties		Nominal %				Carrying Value of Investment (1)
					Ownership
									(in thousands)
	Square 407 Limited Partnership		Market Square North		50	%			$	(8,022	)
	The Metropolitan Square Associates LLC		Metropolitan Square		51	%			8,539
	BP/CRF 901 New York Avenue LLC		901 New York Avenue		25	%		(2)	(1,080		)
	WP Project Developer LLC		Wisconsin Place Land and Infrastructure		33.3	%		(3)	45,514
	Annapolis Junction NFM, LLC		Annapolis Junction		50	%		-4	25,246
	540 Madison Venture LLC		540 Madison Avenue		60	%			68,128
	500 North Capitol LLC		500 North Capitol Street, NW		30	%			(2,250		)
	501 K Street LLC		1001 6th Street (formerly 501 K Street)		50	%		-5	41,736
	Podium Developer LLC		North Station (Phase I - Air Rights)		50	%			4,231
									$	182,042
	_______________
	-1	Investments with deficit balances aggregating approximately $11.4 million have been reflected within Other Liabilities on the Company's Consolidated Balance Sheets.
	-2	The Company’s economic ownership has increased based on the achievement of certain return thresholds.
	-3	The Company’s wholly-owned entity that owns the office component of the project also owns a 33.3% interest in the entity owning the land, parking garage and infrastructure of the project.
	-4	The joint venture owns two in-service buildings, two buildings under construction and two undeveloped land parcels.
	-5	Under the joint venture agreement, the partner will be entitled to up to two additional payments from the venture based on increases in total square footage of the project above 520,000 square feet and achieving certain project returns at stabilization.
	Certain of the Company’s unconsolidated joint venture agreements include provisions whereby, at certain specified times, each partner has the right to initiate a purchase or sale of its interest in the joint ventures at an agreed upon fair value. Under these provisions, the Company is not compelled to purchase the interest of its outside joint venture partners.
	The combined summarized balance sheets of the Company's unconsolidated joint ventures are as follows:
		December 31,				December 31,
		2014				2013
		(in thousands)
	ASSETS
	Real estate and development in process, net	$	1,034,552			$	924,297
	Other assets	264,097				163,149
	Total assets	$	1,298,649			$	1,087,446
	LIABILITIES AND MEMBERS’/PARTNERS’ EQUITY
	Mortgage and notes payable	$	830,075			$	749,732
	Other liabilities	34,211				28,830
	Members’/Partners’ equity	434,363				308,884
	Total liabilities and members’/partners’ equity	$	1,298,649			$	1,087,446
	Company’s share of equity	$	209,828			$	154,726
	Basis differentials (1)	(27,786		)		(28,642		)
	Carrying value of the Company’s investments in unconsolidated joint ventures (2)	$	182,042			$	126,084
	_______________
	-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.
	-2	Investments with deficit balances aggregating approximately $11.4 million and $14.0 million at December 31, 2014 and 2013, respectively, have been reflected within Other Liabilities on the Company's Consolidated Balance Sheets.
	The combined summarized statements of operations of the Company's joint ventures are as follows:
		For the year ended December 31,
		2014				2013			2012
		(in thousands)
	Total revenue (1)	$	158,161			$	311,548		$	564,205
	Expenses
	Operating	62,974				105,319			162,665
	Depreciation and amortization	37,041				86,088			163,134
	Total expenses	100,015				191,407			325,799
	Operating income	58,146				120,141			238,406
	Other income (expense)
	Interest expense	(31,896		)		(112,535		)	(224,645		)
	Losses from early extinguishments of debt	—				(1,677		)	—
	Income from continuing operations	26,250				5,929			13,761
	Gains on sales of real estate	—				14,207			990
	Net income	$	26,250			$	20,136		$	14,751
	Company’s share of net income	$	11,913			$	4,612		$	6,863
	Gains on sales of real estate	—				54,501			—
	Basis differential	856				(1,017		)	1,732
	Elimination of inter-entity interest on partner loan	—				16,978			40,483
	Income from unconsolidated joint ventures	$	12,769			$	75,074		$	49,078
	Gains on consolidation of joint ventures	$	—			$	385,991		$	—
	_______________
-1	Includes straight-line rent adjustments of $3.0 million, $7.8 million and $12.0 million for the years ended December 31, 2014, 2013 and 2012, respectively. Includes net above-/below-market rent adjustments of $(0.1) million, $33.7 million and $91.1 million for the years ended December 31, 2014, 2013 and 2012, respectively. Total revenue for the year ended December 31, 2012 includes termination income totaling approximately $19.6 million (of which the Company's share is approximately $11.8 million) related to a lease termination with a tenant at 767 Fifth Avenue (The General Motors Building).
On April 10, 2014, the Company entered into a joint venture with an unrelated third party to acquire a parcel of land located at 1001 6th Street (formerly 501 K Street) in Washington, DC. The Company anticipates the land parcel will accommodate an approximate 520,000 square foot Class A office property to be developed in the future. The joint venture partner contributed the land for a 50% interest in the joint venture and the Company initially contributed cash of approximately $39.0 million for its 50% interest. Under the joint venture agreement, the partner may be entitled to up to two additional payments from the venture based on increases in total square footage of the project above 520,000 square feet and achieving certain project returns at stabilization.
On April 30, 2014, the Company's partner in its Annapolis Junction joint venture contributed a parcel of land and improvements and the Company contributed cash of approximately $5.4 million to the joint venture. The Company has a 50% interest in this joint venture. The joint venture has commenced construction of Annapolis Junction Building Eight, which when completed will consist of a Class A office property with approximately 125,000 net rentable square feet located in Annapolis, Maryland. In addition, on June 23, 2014, the joint venture obtained construction financing collateralized by the development project totaling $26.0 million. The construction financing bears interest at a variable rate equal to LIBOR plus 1.50% per annum and matures on June 23, 2017, with two, one-year extension options, subject to certain conditions.
On October 24, 2014, a joint venture in which the Company has a 50% interest extended the loan collateralized by its Annapolis Junction Building Six property. At the time of the extension, the outstanding balance of the construction loan totaled approximately $13.9 million and bore interest at a variable rate equal to LIBOR plus 1.65% per annum and was scheduled to mature on November 17, 2014. The extended loan has a total commitment amount of $16.4 million, bears interest at a variable rate equal to LIBOR plus 2.25% per annum and matures on November 17, 2015. Annapolis Junction Building Six is a Class A office property with approximately 119,000 net rentable square feet located in Annapolis, Maryland.
On December 17, 2014, a joint venture in which the Company has a 25% nominal ownership interest refinanced with a new lender its mortgage loan collateralized by 901 New York Avenue located in Washington, DC. The mortgage loan totaling approximately $150.4 million bore interest at a fixed rate of 5.19% per annum and was scheduled to mature on January 1, 2015. The new mortgage loan totaling $225.0 million bears interest at a fixed rate of 3.61% per annum and matures on January 5, 2025.
On December 19, 2014, the Company entered into a joint venture with an unrelated third party to acquire the air rights for the future development of the first phase at North Station, consisting of an atrium hall and podium building containing up to 377,000 net rentable square feet of retail and office space located in Boston, Massachusetts. The joint venture partner contributed air rights parcels and improvements, with a fair value of approximately $13.0 million, for its initial 50% interest in the joint venture. The Company contributed improvements totaling approximately $4.2 million and will contribute cash totaling approximately $8.8 million for its initial 50% interest. In addition, the Company entered into an option and development rights agreement with its partner pursuant to which the Company has the right to develop residential, hotel and office space in future phases, subject to certain terms and conditions including the partner's right to participate as a venture partner in each phase of the project.

Mortgage_Notes_Payable

Mortgage Notes Payable	12 Months Ended
	Dec. 31, 2014
Loans Payable [Abstract]
Mortgage Notes Payable	Mortgage Notes Payable
	The Company had outstanding mortgage notes payable totaling approximately $4.3 billion and $4.4 billion as of December 31, 2014 and 2013, respectively, each collateralized by one or more buildings and related land included in real estate assets. The mortgage notes payable are generally due in monthly installments and mature at various dates through April 10, 2022.
	Fixed rate mortgage notes payable totaled approximately $4.3 billion and $4.4 billion at December 31, 2014 and 2013, respectively, with contractual interest rates ranging from 4.75% to 7.69% per annum at December 31, 2014 and 2013 (with a weighted-average of 5.70% (excluding the mezzanine notes payable) at December 31, 2014 and 2013).
	There were no variable rate mortgage loans at December 31, 2014 and 2013. As of December 31, 2014 and 2013, the LIBOR rate was 0.17%.
	On July 1, 2014, the Company used available cash to repay the mortgage loan collateralized by its New Dominion Technology Park Building Two property located in Herndon, Virginia totaling $63.0 million. The mortgage loan bore interest at a fixed rate of 5.55% per annum and was scheduled to mature on October 1, 2014. There was no prepayment penalty.
	Four mortgage loans totaling approximately $2.2 billion at December 31, 2014 and 2013 have been accounted for at their fair values on the dates the mortgage loans were assumed. The impact of recording the mortgage loans at fair value resulted in a decrease to interest expense of approximately $52.5 million, $34.4 million and $7.0 million for the years ended December 31, 2014, 2013 and 2012, respectively. The cumulative liability related to the fair value adjustments was $138.7 million and $191.2 million at December 31, 2014 and 2013, respectively, and is included in mortgage notes payable in the Consolidated Balance Sheets.
	Contractual aggregate principal payments of mortgage notes payable at December 31, 2014 are as follows:
		Principal Payments
		(in thousands)
	2015	$	26,184
	2016	608,879
	2017	2,821,750
	2018	18,633
	2019	19,670
	Thereafter	675,657
	Total aggregate principal payments	4,170,773
	Unamortized balance of historical fair value adjustments	138,711
	Total carrying value of mortgage notes payable	$	4,309,484

Unsecured_Senior_Notes

Unsecured Senior Notes	12 Months Ended
	Dec. 31, 2014
Debt Disclosure [Abstract]
Unsecured Senior Notes	Unsecured Senior Notes
	The following summarizes the unsecured senior notes outstanding as of December 31, 2014 (dollars in thousands):
		Coupon/		Effective		Principal			Maturity Date(2)
		Stated Rate		Rate(1)		Amount
	10 Year Unsecured Senior Notes	5.875	%	5.967	%	$	700,000		October 15, 2019
	10 Year Unsecured Senior Notes	5.625	%	5.708	%	700,000			November 15, 2020
	10 Year Unsecured Senior Notes	4.125	%	4.289	%	850,000			May 15, 2021
	7 Year Unsecured Senior Notes	3.7	%	3.853	%	850,000			November 15, 2018
	11 Year Unsecured Senior Notes	3.85	%	3.954	%	1,000,000			February 1, 2023
	10.5 Year Unsecured Senior Notes	3.125	%	3.279	%	500,000			September 1, 2023
	10.5 Year Unsecured Senior Notes	3.8	%	3.916	%	700,000			February 1, 2024
	Total principal					5,300,000
	Net unamortized discount					(12,296		)
	Total					$	5,287,704
	_______________
	-1	Yield on issuance date including the effects of discounts on the notes and the amortization of financing costs.
	-2	No principal amounts are due prior to maturity.
	On December 15, 2014, the Company’s Operating Partnership used available cash to redeem $300.0 million in aggregate principal amount of its 5.625% senior notes due 2015 (the “5.625% Notes”) and $250.0 million in aggregate principal amount of its 5.000% senior notes due 2015 (the “5.000% Notes”). The redemption price for the 5.625% Notes was determined in accordance with the applicable indenture and totaled approximately $308.0 million. The redemption price included approximately $2.8 million of accrued and unpaid interest to, but not including, the redemption date. Excluding such accrued and unpaid interest, the redemption price was approximately 101.73% of the principal amount being redeemed. The redemption price for the 5.000% Notes was determined in accordance with the applicable indenture and totaled approximately $255.8 million. The redemption price included approximately $0.5 million of accrued and unpaid interest to, but not including, the redemption date. Excluding such accrued and unpaid interest, the redemption price was approximately 102.13% of the principal amount being redeemed. The Company recognized a loss on early extinguishment of debt totaling approximately $10.6 million, which amount included the payment of the redemption premium totaling approximately $10.5 million.
	The indenture relating to the unsecured senior notes contains certain financial restrictions and requirements, including (1) a leverage ratio not to exceed 60%, (2) a secured debt leverage ratio not to exceed 50%, (3) an interest coverage ratio of greater than 1.50, and (4) an unencumbered asset value of not less than 150% of unsecured debt. At December 31, 2014, the Company was in compliance with each of these financial restrictions and requirements.

Unsecured_Exchangeable_Senior_

Unsecured Exchangeable Senior Notes	12 Months Ended
	Dec. 31, 2014
Debt Disclosure [Abstract]
Unsecured Exchangeable Senior Notes	Unsecured Exchangeable Senior Notes
	ASC 470-20 "Debt with Conversion and Other Options" ("ASC 470-20") requires the liability and equity components of convertible debt instruments that may be settled in cash upon conversion (including partial cash settlement) to be separately accounted for in a manner that reflects the issuer’s nonconvertible debt borrowing rate. ASC 470-20 requires that the initial proceeds from the sale of the Operating Partnership’s $862.5 million of 2.875% exchangeable senior notes due 2037 (all of which had been redeemed/repurchased as of December 31, 2012), $450.0 million of 3.75% exchangeable senior notes due 2036 (all of which have been redeemed/repurchased as of December 31, 2013) and $747.5 million of 3.625% exchangeable senior notes due 2014 (all of which have been repaid as of December 31, 2014) be allocated between a liability component and an equity component in a manner that reflects interest expense at the interest rate of similar nonconvertible debt that could have been issued by the Operating Partnership at such time. The Company measured the fair value of the debt components of the 2.875%, 3.75% and 3.625% exchangeable senior notes for the periods presented based on effective interest rates of 5.630%, 5.958% and 6.555%, respectively. The aggregate carrying amount of the debt component was approximately $0.0 million and $744.9 million (net of the equity component allocation adjustment of approximately $0.0 million and $2.4 million) at December 31, 2014 and December 31, 2013, respectively. As a result, the Company attributed an aggregate of approximately $230.3 million of the proceeds to the equity component of the notes, which represents the excess proceeds received over the fair value of the notes at the date of issuance. The equity component of the notes has been reflected within Additional Paid-in Capital in the Consolidated Balance Sheets. The Company reclassified approximately $1.0 million of deferred financing costs to Additional Paid-in Capital, which represented the costs attributable to the equity components of the notes. The carrying amount of the equity component was approximately $0.0 million and $91.9 million at December 31, 2014 and December 31, 2013, respectively. The resulting debt discount has been amortized over the period during which the debt was expected to be outstanding (i.e., through the first optional redemption dates or, in the case of the 2014 notes, the maturity date) as additional non-cash interest expense. The aggregate contractual interest expense was approximately $3.3 million, $34.8 million and $48.4 million for the years ended December 31, 2014, 2013 and 2012, respectively. As a result, the Company reported additional non-cash interest expense of approximately $2.4 million, $23.1 million and $29.1 million for the years ended December 31, 2014, 2013 and 2012, respectively.
	On February 18, 2014, the Company's Operating Partnership repaid at maturity the $747.5 million aggregate principal amount of its 3.625% exchangeable senior notes due 2014 plus accrued and unpaid interest thereon.

Unsecured_Line_of_Credit

Unsecured Line of Credit	12 Months Ended
	Dec. 31, 2014
Unsecured Line of Credit [Abstract]
Unsecured Line of Credit [Text Block]	9.    Unsecured Line of Credit
	The Company's Operating Partnership has a $1.0 billion revolving credit facility (the "Unsecured Line of Credit") with a maturity date of July 26, 2018. The Operating Partnership may increase the total commitment to $1.5 billion, subject to syndication of the increase and other conditions. At the Operating Partnership’s option, loans outstanding under the Unsecured Line of Credit will bear interest at a rate per annum equal to (1), in the case of loans denominated in Dollars, Euro or Sterling, LIBOR or, in the case of loans denominated in Canadian Dollars, CDOR, in each case, plus a margin ranging from 0.925% to 1.70% based on the Operating Partnership’s credit rating or (2) an alternate base rate equal to the greatest of (a) the Administrative Agent’s prime rate, (b) the Federal Funds rate plus 0.5% or (c) LIBOR for a one month period plus 1.00%, in each case, plus a margin ranging from 0.0% to 0.70% based on the Operating Partnership’s credit rating. The Unsecured Line of Credit also contains a competitive bid option that allows banks that are part of the lender consortium to bid to make loan advances to the Operating Partnership at a reduced interest rate. In addition, the Operating Partnership is also obligated to pay (1) in quarterly installments a facility fee on the total commitment at a rate per annum ranging from 0.125% to 0.35% based on the Operating Partnership’s credit rating and (2) an annual fee on the undrawn amount of each letter of credit equal to the LIBOR margin. Based on the Operating Partnership’s current credit rating, the LIBOR and CDOR margin is 1.00%, the alternate base rate margin is 0.0% and the facility fee is 0.15%. At December 31, 2014 and 2013, there were no amounts outstanding on the Unsecured Line of Credit.
	The terms of the Unsecured Line of Credit require that the Company maintain a number of customary financial and other covenants on an ongoing basis, including: (1) a leverage ratio not to exceed 60%, however, the leverage ratio may increase to no greater than 65% provided that it is reduced back to 60% within one year, (2) a secured debt leverage ratio not to exceed 55%, (3) a fixed charge coverage ratio of at least 1.40, (4) an unsecured debt leverage ratio not to exceed 60%, however, the unsecured debt leverage ratio may increase to no greater than 65% provided that it is reduced back to 60% within one year, (5) an unsecured debt interest coverage ratio of at least 1.75 and (6) limitations on permitted investments. At December 31, 2014, the Company was in compliance with each of these financial and other covenant requirements.

Commitments_And_Contingencies

Commitments And Contingencies	12 Months Ended
	Dec. 31, 2014
Commitments and Contingencies Disclosure [Abstract]
Commitments And Contingencies	Commitments and Contingencies
	General
	In the normal course of business, the Company guarantees its performance of services or indemnifies third parties against its negligence. In addition, in the normal course of business, the Company guarantees to certain tenants the obligations of its subsidiaries for the payment of tenant improvement allowances and brokerage commissions in connection with their leases and limited costs arising from delays in delivery of their premises.
	The Company has letter of credit and performance obligations of approximately $19.8 million related to lender and development requirements.
	Certain of the Company’s joint venture agreements include provisions whereby, at certain specified times, each partner has the right to initiate a purchase or sale of its interest in the joint ventures. With limited exception, under these provisions, the Company is not compelled to purchase the interest of its outside joint venture partners (See also Note 11). Under certain of the Company's joint venture agreements, if certain return thresholds are achieved the partners will be entitled to an additional promoted interest or payments.
	In connection with the assumption of 767 Fifth Avenue's (the General Motors Building) secured loan by the Company’s consolidated joint venture, 767 Venture, LLC, the Company guaranteed the consolidated joint venture’s obligation to fund various escrows, including tenant improvements, taxes and insurance in lieu of cash deposits. As of December 31, 2014, the maximum funding obligation under the guarantee was approximately $32.0 million. The Company earns a fee from the joint venture for providing the guarantee and has an agreement with the outside partners to reimburse the joint venture for their share of any payments made under the guarantee.
	In connection with the mortgage financing collateralized by the Company's John Hancock Tower property located in Boston, Massachusetts, the Company has agreed to guarantee approximately $25.7 million related to its obligation to provide funds for certain tenant re-leasing costs. The mortgage financing matures on January 6, 2017.
	In connection with the mortgage financing collateralized by the Company's consolidated joint venture's Fountain Square property located in Reston, Virginia, the Company has agreed to guarantee approximately $0.7 million related to its obligation to provide funds for certain tenant re-leasing costs. The mortgage financing matures on October 11, 2016.
	From time to time, the Company (or the applicable joint venture) has also agreed to guarantee portions of the principal, interest or other amounts in connection with other unconsolidated joint venture borrowings. In addition to the financial guarantees referenced above, the Company has agreed to customary construction completion guarantees for construction loans, environmental indemnifications and nonrecourse carve-outs (e.g., guarantees against fraud, misrepresentation and bankruptcy) on certain of its unconsolidated joint venture loans.
	In 2009, the Company filed a general unsecured creditor’s claim against Lehman Brothers, Inc. for approximately $45.3 million related to its rejection of a lease at 399 Park Avenue in New York City. On January 10, 2014, the trustee for the liquidation of the business of Lehman Brothers allowed the Company’s claim in the amount of approximately $45.2 million. On September 18, 2014, the Company received an initial distribution totaling approximately $7.7 million, which is included in Base Rent in the accompanying Consolidated Statements of Operations for the year ended December 31, 2014, leaving a remaining claim of approximately $37.5 million. Recently, claims of similar priority to that of the Company's remaining claim were quoted privately within a range of $0.24 to $0.25 per $1.00. The Company was notified on February 19, 2015 that the bankruptcy court approved the trustee’s motion to make a second interim distribution to holders of claims as of February 6, 2015. The Company will continue to evaluate whether to attempt to sell the remaining claim or wait until the trustee distributes proceeds from the Lehman estate. Given the inherent uncertainties in bankruptcy proceedings, there can be no assurance as to the timing or amount of proceeds, if any, that the Company may ultimately realize on the remaining claim, whether by sale to a third party or by one or more distributions from the trustee. Accordingly, the Company has not recorded any estimated recoveries associated with this gain contingency within its consolidated financial statements at December 31, 2014.
	Concentrations of Credit Risk
	Management of the Company performs ongoing credit evaluations of tenants and may require tenants to provide some form of credit support such as corporate guarantees and/or other financial guarantees. Although the Company’s properties are geographically diverse and the tenants operate in a variety of industries, to the extent the Company has a significant concentration of rental revenue from any single tenant, the inability of that tenant to make its lease payments could have an adverse effect on the Company.
	Some potential losses are not covered by insurance.
	The Company carries insurance coverage on its properties of types and in amounts and with deductibles that it believes are in line with coverage customarily obtained by owners of similar properties. In response to the uncertainty in the insurance market following the terrorist attacks of September 11, 2001, the Federal Terrorism Risk Insurance Act (as amended, “TRIA”) was enacted in November 2002 to require regulated insurers to make available coverage for “certified” acts of terrorism (as defined by the statute). The expiration date of TRIA was extended to December 31, 2014 by the Terrorism Risk Insurance Program Reauthorization Act of 2007 and further extended to December 31, 2020 by the Terrorism Risk Insurance Program Reauthorization Act of 2015 (“TRIPRA”), and the Company can provide no assurance that it will be extended further. Currently, the Company’s property insurance program per occurrence limits are $1.0 billion for its portfolio insurance program, including coverage for acts of terrorism other than nuclear, biological, chemical or radiological terrorism (“Terrorism Coverage”). The Company also carries $250 million of Terrorism Coverage for 601 Lexington Avenue, New York, New York (“601 Lexington Avenue”) in excess of the $1.0 billion of coverage in the Company’s property insurance program. Certain properties, including the General Motors Building located at 767 Fifth Avenue in New York, New York (“767 Fifth Avenue”), are currently insured in separate insurance programs. The property insurance program per occurrence limits for 767 Fifth Avenue are $1.625 billion, including Terrorism Coverage. Through June 9, 2014, $1.375 billion of the Terrorism Coverage for 767 Fifth Avenue in excess of $250 million was provided by NYXP, LLC (“NYXP”), as a direct insurer. After June 9, 2014, all of the Terrorism Coverage for 767 Fifth Avenue has been provided by third party insurers. The Company also currently carries nuclear, biological, chemical and radiological terrorism insurance coverage for acts of terrorism certified under TRIA (“NBCR Coverage”), which is provided by IXP as a direct insurer, for the properties in our portfolio, including 767 Fifth Avenue, but excluding certain other properties owned in joint ventures with third parties or which the Company manages. The per occurrence limit for NBCR Coverage is $1.0 billion. Under TRIA, after the payment of the required deductible and coinsurance, the NBCR Coverage provided by IXP and the Terrorism Coverage provided by NYXP are backstopped by the Federal Government if the aggregate industry insured losses resulting from a certified act of terrorism exceed a “program trigger.” In 2015, the program trigger is $100 million and the coinsurance is 15%, however, both will increase in subsequent years pursuant to TRIPRA. If the Federal Government pays out for a loss under TRIA, it is mandatory that the Federal Government recoup the full amount of the loss from insurers offering TRIA coverage after the payment of the loss pursuant to a formula in TRIPRA. The Company may elect to terminate the NBCR Coverage if the Federal Government seeks recoupment for losses paid under TRIA, if there is a change in its portfolio or for any other reason. The Company intends to continue to monitor the scope, nature and cost of available terrorism insurance and maintain terrorism insurance in amounts and on terms that are commercially reasonable.
	The Company also currently carries earthquake insurance on its properties located in areas known to be subject to earthquakes in an amount and subject to self-insurance that the Company believes is commercially reasonable. In addition, this insurance is subject to a deductible in the amount of 5% of the value of the affected property. Specifically, the Company currently carries earthquake insurance which covers its San Francisco region (excluding 535 Mission Street and Salesforce Tower) with a $120 million per occurrence limit and a $120 million annual aggregate limit, $20 million of which is provided by IXP, as a direct insurer. The builders risk policy maintained for the development of 535 Mission Street in San Francisco included a $15 million per occurrence and annual aggregate limit of earthquake coverage through October 22, 2014, after which time 535 Mission Street was included in our portfolio earthquake insurance program. In addition, the builders risk policy maintained for the development of Salesforce Tower in San Francisco includes a $60 million per occurrence and annual aggregate limit of earthquake coverage (increased from $15 million on July 29, 2014). The amount of the Company’s earthquake insurance coverage may not be sufficient to cover losses from earthquakes. In addition, the amount of earthquake coverage could impact the Company’s ability to finance properties subject to earthquake risk. The Company may discontinue earthquake insurance or change the structure of its earthquake insurance program on some or all of its properties in the future if the premiums exceed the Company’s estimation of the value of the coverage.
	IXP, a captive insurance company which is a wholly-owned subsidiary of the Company, acts as a direct insurer with respect to a portion of the Company’s earthquake insurance coverage for its Greater San Francisco properties and the Company’s NBCR Coverage. NYXP, a captive insurance company which is a wholly-owned subsidiary of the Company, acted as a direct insurer with respect to a portion of the Company’s Terrorism Coverage for 767 Fifth Avenue through June 9, 2014. NYXP only insured losses which exceeded the program trigger under TRIA and NYXP reinsured with a third-party insurance company any coinsurance payable under TRIA. Insofar as the Company owns IXP and NYXP, it is responsible for their liquidity and capital resources, and the accounts of IXP and NYXP are part of the Company’s consolidated financial statements. In particular, if a loss occurs which is covered by the Company’s NBCR Coverage but is less than the applicable program trigger under TRIA, IXP would be responsible for the full amount of the loss without any backstop by the Federal Government. IXP and NYXP would also be responsible for any recoupment charges by the Federal Government in the event losses are paid out and their insurance policies are maintained after the payout by the Federal Government. If the Company experiences a loss and IXP or NYXP are required to pay under their insurance policies, the Company would ultimately record the loss to the extent of the required payment. Therefore, insurance coverage provided by IXP and NYXP should not be considered as the equivalent of third-party insurance, but rather as a modified form of self-insurance. In addition, the Operating Partnership has issued a guarantee to cover liabilities of IXP in the amount of $20.0 million.
	The mortgages on the Company's properties typically contain requirements concerning the financial ratings of the insurers who provide policies covering the property. The Company provides the lenders on a regular basis with the identity of the insurance companies in the Company's insurance programs. The ratings of some of the Company's insurers are below the rating requirements in some of the Company's loan agreements and the lenders for these loans could attempt to claim that an event of default has occurred under the loan. The Company believes it could obtain insurance with insurers which satisfy the rating requirements. Additionally, in the future, the Company's ability to obtain debt financing secured by individual properties, or the terms of such financing, may be adversely affected if lenders generally insist on ratings for insurers or amounts of insurance which are difficult to obtain or which result in a commercially unreasonable premium. There can be no assurance that a deficiency in the financial ratings of one or more of the Company's insurers will not have a material adverse effect on the Company.
	The Company continues to monitor the state of the insurance market in general, and the scope and costs of coverage for acts of terrorism and California earthquake risk in particular, but the Company cannot anticipate what coverage will be available on commercially reasonable terms in future policy years. There are other types of losses, such as from wars, for which the Company cannot obtain insurance at all or at a reasonable cost. With respect to such losses and losses from acts of terrorism, earthquakes or other catastrophic events, if the Company experiences a loss that is uninsured or that exceeds policy limits, the Company could lose the capital invested in the damaged properties, as well as the anticipated future revenues from those properties. Depending on the specific circumstances of each affected property, it is possible that the Company could be liable for mortgage indebtedness or other obligations related to the property. Any such loss could materially and adversely affect the Company's business and financial condition and results of operations.
	Legal Matters
	The Company is subject to various legal proceedings and claims that arise in the ordinary course of business. These matters are generally covered by insurance. Management believes that the final outcome of such matters will not have a material adverse effect on the financial position, results of operations or liquidity of the Company.
	State and Local Tax Matters
	Because the Company is organized and qualifies as a REIT, it is generally not subject to federal income taxes, but is subject to certain state and local taxes. In the normal course of business, certain entities through which the Company owns real estate either have undergone, or are currently undergoing, tax audits. Although the Company believes that it has substantial arguments in favor of its positions in the ongoing audits, in some instances there is no controlling precedent or interpretive guidance on the specific point at issue. Collectively, tax deficiency notices received to date from the jurisdictions conducting the ongoing audits have not been material. However, there can be no assurance that future audits will not occur with increased frequency or that the ultimate result of such audits will not have a material adverse effect on the Company’s results of operations.
	Environmental Matters
	It is the Company’s policy to retain independent environmental consultants to conduct or update Phase I environmental assessments (which generally do not involve invasive techniques such as soil or ground water sampling) and asbestos surveys in connection with the Company’s acquisition of properties. These pre-purchase environmental assessments have not revealed environmental conditions that the Company believes will have a material adverse effect on its business, assets, financial condition, results of operations or liquidity, and the Company is not otherwise aware of environmental conditions with respect to its properties that the Company believes would have such a material adverse effect. However, from time to time environmental conditions at the Company’s properties have required and may in the future require environmental testing and/or regulatory filings, as well as remedial action.
	In February 1999, the Company (through a joint venture) acquired from Exxon Corporation a property in Massachusetts that was formerly used as a petroleum bulk storage and distribution facility and was known by the state regulatory authority to contain soil and groundwater contamination. The Company developed an office park on the property. The Company engaged a specially licensed environmental consultant to oversee the management of contaminated soil and groundwater that was disturbed in the course of construction. Under the property acquisition agreement, Exxon agreed to (1) bear the liability arising from releases or discharges of oil and hazardous substances which occurred at the site prior to the Company’s ownership, (2) continue monitoring and/or remediating such releases and discharges as necessary and appropriate to comply with applicable requirements, and (3) indemnify the Company for certain losses arising from preexisting site conditions. Any indemnity claim may be subject to various defenses, and there can be no assurance that the amounts paid under the indemnity, if any, would be sufficient to cover the liabilities arising from any such releases and discharges.
	Environmental investigations at some of the Company’s properties and certain properties owned by affiliates of the Company have identified groundwater contamination migrating from off-site source properties. In each case the Company engaged a licensed environmental consultant to perform the necessary investigations and assessments and to prepare any required submittals to the regulatory authorities. In each case the environmental consultant concluded that the properties qualify under the regulatory program or the regulatory practice for a status which eliminates certain deadlines for conducting response actions at a site. The Company also believes that these properties qualify for liability relief under certain statutory provisions or regulatory practices regarding upgradient releases. Although the Company believes that the current or former owners of the upgradient source properties may bear responsibility for some or all of the costs of addressing the identified
	groundwater contamination, the Company will take such further response actions (if any) that it deems necessary or advisable. Other than periodic testing at some of these properties, no such additional response actions are anticipated at this time.
	Some of the Company’s properties and certain properties owned by the Company’s affiliates are located in urban, industrial and other previously developed areas where fill or current or historical uses of the areas have caused site contamination. Accordingly, it is sometimes necessary to institute special soil and/or groundwater handling procedures and/or include particular building design features in connection with development, construction and other property operations in order to achieve regulatory closure and/or ensure that contaminated materials are addressed in an appropriate manner. In these situations it is the Company’s practice to investigate the nature and extent of detected contamination and estimate the costs of required response actions and special handling procedures. The Company then uses this information as part of its decision-making process with respect to the acquisition and/or development of the property. For example, the Company owns a parcel in Massachusetts which was formerly used as a quarry/asphalt batching facility. Pre-purchase testing indicated that the site contained relatively low levels of certain contaminants. The Company has developed an office park on this property. Prior to and during redevelopment activities, the Company engaged a specially licensed environmental consultant to monitor environmental conditions at the site and prepare necessary regulatory submittals based on the results of an environmental risk characterization. A submittal has been made to the regulatory authorities in order to achieve regulatory closure at this site. The submittal included an environmental deed restriction that mandates compliance with certain protective measures in a portion of the site where low levels of residual soil contamination have been left in place in accordance with applicable laws.
	The Company expects that resolution of the environmental matters relating to the above will not have a material impact on its business, assets, financial condition, results of operations or liquidity. However, the Company cannot assure you that it has identified all environmental liabilities at its properties, that all necessary remediation actions have been or will be undertaken at the Company’s properties or that the Company will be indemnified, in full or at all, in the event that such environmental liabilities arise.
	Tax Protection Obligations
	In connection with the acquisition or contribution of three properties, the Company entered into agreements for the benefit of the selling or contributing parties which specifically state that until such time as the contributors do not hold at least a specified percentage of the OP Units owned by such person following the contribution of the properties, or until June 9, 2017 for 767 Fifth Avenue (the General Motors Building), the Operating Partnership will not sell or otherwise transfer the properties in a taxable transaction. If the Company does sell or transfer the properties in a taxable transaction, it would be liable to the contributors for contractual damages.

Noncontrolling_Interests

Noncontrolling Interests	12 Months Ended
	Dec. 31, 2014
Noncontrolling Interest [Abstract]
Noncontrolling Interests	Noncontrolling Interests
	Noncontrolling interests relate to the interests in the Operating Partnership not owned by the Company and interests in consolidated property partnerships not wholly-owned by the Company. As of December 31, 2014, the noncontrolling interests in the Operating Partnership consisted of 16,453,670 OP Units, 1,496,799 LTIP Units, 394,590 2012 OPP Units, 313,936 2013 MYLTIP Units, 482,032 2014 MYLTIP Units and 12,667 Series Four Preferred Units (none of which are convertible into OP Units) held by parties other than the Company.
	Noncontrolling Interest—Redeemable Preferred Units of the Operating Partnership
	On March 11, 2014, the Company notified the holders of the outstanding Series Two Preferred Units that it had elected to redeem all of such Series Two Preferred Units on May 12, 2014. As a result of the Company's election to redeem the units, as of May 12, 2014, the holders of all remaining 666,116 Series Two Preferred Units converted such units into an aggregate of 874,168 OP Units. The Series Two Preferred Units bore a preferred distribution equal to the greater of (1) the distribution which would have been paid in respect of the Series Two Preferred Unit had such Series Two Preferred Unit been converted into an OP Unit (including both regular and special distributions) or (2) 6.00% per annum on a liquidation preference of $50.00 per unit, and were convertible into OP Units at a rate of $38.10 per Preferred Unit (1.312336 OP Units for each Preferred Unit). In connection with the conversion of the remaining Series Two Preferred Units in May 2014, the Operating Partnership paid accrued and unpaid distributions which included the special cash distribution on an as-converted basis. On February 18, 2014, the Operating Partnership paid a distribution on its outstanding Series Two Preferred Units of $0.85302 per unit. Due to the holders' redemption option existing outside the control of the Company, the Series Two Preferred Units were presented outside of permanent equity in the Company's Consolidated Balance Sheets.
	During the year ended December 31, 2013, 329,881 Series Two Preferred Units of the Operating Partnership were converted by the holders into 432,914 OP Units. In addition, the Company paid the accrued preferred distributions due to the holders of Preferred Units that were converted.
	The Preferred Units at December 31, 2014 consisted of 12,667 Series Four Preferred Units, which bear a preferred distribution equal to 2.00% per annum on a liquidation preference of $50.00 per unit and are not convertible into OP Units. The holders of Series Four Preferred Units have the right, at certain times and subject to certain conditions set forth in the Certificate of Designations establishing the rights, limitations and preferences of the Series Four Preferred Units, to require the Operating Partnership to redeem all of their units for cash at the redemption price of $50.00 per unit. The Operating Partnership also has the right, at certain times and subject to certain conditions, to redeem all of the Series Four Preferred Units for cash at the redemption price of $50.00 per unit. In order to secure the performance of certain post-issuance obligations by the holders, all of such outstanding Series Four Preferred Units were subject to forfeiture pursuant to the terms of a pledge agreement and not eligible for redemption until and unless such security interest is released. The Operating Partnership's first right to redeem the Series Four Preferred Units was a 30-day period beginning on August 29, 2013. On August 29, 2013, the Company's Operating Partnership redeemed approximately 861,400 Series Four Preferred Units for cash at the redemption price of $50.00 per unit plus accrued and unpaid distributions through the redemption date. On May 19, 2014, the Company's Operating Partnership released to the holders 319,687 Series Four Preferred Units that were previously subject to the security interest. On July 3, 2014, the Company's Operating Partnership redeemed such units for cash totaling approximately $16.0 million, plus accrued and unpaid distributions. On October 16, 2014, the Company's Operating Partnership released to the holders 27,773 Series Four Preferred Units that were previously subject to the security interest under the pledge agreement. On November 5, 2014, the Company's Operating Partnership redeemed such units for cash totaling approximately $1.4 million. An aggregate of 12,667 Series Four Preferred Units remain outstanding and subject to the security interest under the pledge agreement. Due to the holders' redemption option existing outside the control of the Company, the Series Four Preferred Units are presented outside of permanent equity in the Company's Consolidated Balance Sheets.
	On February 18, 2014, the Operating Partnership paid a distribution on its outstanding Series Four Preferred Units of $0.25 per unit. On May 15, 2014, the Operating Partnership paid a distribution on its outstanding Series Four Preferred Units of $0.25 per unit. On August 15, 2014, the Operating Partnership paid a distribution on its outstanding Series Four Preferred Units of $0.25 per unit. On November 17, 2014, the Operating Partnership paid a distribution on its outstanding Series Four Preferred Units of $0.25 per unit.
	The following table reflects the activity of the noncontrolling interests—redeemable preferred units of the Operating Partnership for the years ended December 31, 2014, 2013 and 2012 (in thousands):
	Balance at December 31, 2011	$	55,652
	Issuance of redeemable preferred units (Series Four Preferred Units)	79,405
	Net income	3,497
	Distributions	(3,497		)
	Redemption of redeemable preferred units (Series Four Preferred Units)	(18,329		)
	Conversion of redeemable preferred units (Series Two Preferred Units) to common units	(5,852		)
	Balance at December 31, 2012	110,876
	Net income	6,046
	Distributions	(6,046		)
	Redemption of redeemable preferred units (Series Four Preferred Units)	(43,070		)
	Conversion of redeemable preferred units (Series Two Preferred Units) to common units	(16,494		)
	Balance at December 31, 2013	51,312
	Net income	1,023
	Distributions	(1,023		)
	Redemption of redeemable preferred units (Series Four Preferred Units)	(17,373		)
	Conversion of redeemable preferred units (Series Two Preferred Units) to common units	(33,306		)
	Balance at December 31, 2014	$	633
	Noncontrolling Interest—Redeemable Interest in Property Partnership
	On October 4, 2012, the Company completed the formation of a joint venture, which owns and operates Fountain Square located in Reston, Virginia. The joint venture partner contributed the property valued at approximately $385.0 million and related mortgage indebtedness totaling approximately $211.3 million for a 50% interest in the joint venture. The Company contributed cash totaling approximately $87.0 million for its 50% interest, which cash was distributed to the joint venture partner. Pursuant to the joint venture agreement (i) the Company has rights to acquire the partner's nominal 50% interest and (ii) the partner has the right to cause the Company to acquire the partner's interest on January 4, 2016, in each case at a fixed price totaling approximately $102.0 million in cash. The fixed price option rights expire on January 31, 2016. The Company is consolidating this joint venture due to the Company's right to acquire the partner's nominal 50% interest. The Company initially recorded the noncontrolling interest at its acquisition-date fair value as temporary equity, due to the redemption option existing outside the control of the Company. The Company will accrete the changes in the redemption value quarterly over the period from the acquisition date to the earliest redemption date using the effective interest method.  The Company will record the accretion after the allocation of net income and distributions of cash flow to the noncontrolling interest account balance.
	The following table reflects the activity of the noncontrolling interest—redeemable interest in property partnership in the Company's Fountain Square consolidated joint venture for the years ended December 31, 2014, 2013 and 2012 (in thousands):
	Balance at December 31, 2011	$	—
	Acquisition-date fair value of redeemable interest	98,787
	Net loss	(719		)
	Distributions	(3,032		)
	Adjustment to reflect redeemable interest at redemption value	2,522
	Balance at December 31, 2012	97,558
	Net loss	(1,839		)
	Distributions	(4,585		)
	Adjustment to reflect redeemable interest at redemption value	8,475
	Balance at December 31, 2013	99,609
	Net loss	(603		)
	Distributions	(6,000		)
	Adjustment to reflect redeemable interest at redemption value	11,686			(1	)
	Balance at December 31, 2014	$	104,692
	_______________
	(1) Includes an out-of-period adjustment totaling approximately $1.9 million (See Note 2).
	Noncontrolling Interest—Common Units of the Operating Partnership
	During the years ended December 31, 2014 and 2013, 80,246 and 929,441 OP Units, respectively, were presented by the holders for redemption (including 3,734 and 432,914 OP Units, respectively, issued upon conversion of Series Two Preferred Units and 67,857 and 24,028 OP Units, respectively, issued upon conversion of LTIP Units) and were redeemed by the Company in exchange for an equal number of shares of Common Stock.
	At December 31, 2014, the Company had outstanding 394,590 2012 OPP Units, 313,936 2013 MYLTIP Units and 482,032 2014 MYLTIP Units (See Note 17). Prior to the measurement date (February 6, 2015 for 2012 OPP Units (See note 20), February 4, 2016 for 2013 MYLTIP Units and February 3, 2017 for 2014 MYLTIP Units), holders of OPP Units and MYLTIP Units are entitled to receive per unit distributions equal to one-tenth (10%) of the regular quarterly distributions payable on an OP Unit, but will not be entitled to receive any special distributions. After the measurement date, the number of OPP Units and MYLTIP Units, both vested and unvested, that OPP and MYLTIP award recipients have earned, if any, based on the establishment of an outperformance pool, will be entitled to receive distributions in an amount per unit equal to distributions, both regular and special, payable on an OP Unit.
	On January 31, 2014, the measurement period for the Company’s 2011 OPP Unit awards expired and the Company’s TRS was not sufficient for employees to earn and therefore become eligible to vest in any of the 2011 OPP Unit awards. As a result, the Company accelerated the then remaining unrecognized compensation expense totaling approximately $1.2 million during the year ended December 31, 2014. Accordingly, all 2011 OPP Unit awards were automatically forfeited.
	On January 29, 2014, the Operating Partnership paid a special cash distribution on the OP Units and LTIP Units in the amount of $2.25 per unit, a regular quarterly cash distribution on the OP Units and LTIP Units in the amount of $0.65 per unit, and a regular quarterly distribution on the 2011 OPP Units, 2012 OPP Units and 2013 MYLTIP Units in the amount of $0.065 per unit, to holders of record as of the close of business on December 31, 2013. The special cash distribution was in addition to the regular quarterly distribution on the OP Units and LTIP Units. Holders of the 2011 OPP Units, 2012 OPP Units, 2013 MYLTIP Units and 2014 MYLTIP Units are not entitled to receive any special distributions. On April 30, 2014, the Operating Partnership paid a distribution on the OP Units and LTIP Units in the amount of $0.65 per unit, and a distribution on the 2012 OPP Units, 2013 MYLTIP Units and 2014 MYLTIP Units in the amount of $0.065 per unit, to holders of record as of the close of business on March 31, 2014. On July 31, 2014, the Operating Partnership paid a distribution on the OP Units and LTIP Units in the amount of $0.65 per unit, and a distribution on the 2012 OPP Units, 2013 MYLTIP Units and 2014 MYLTIP Units in the amount of $0.065 per unit, to holders of record as of the close of business on June 30, 2014. On October 31, 2014, the Operating Partnership paid a distribution on the OP Units and LTIP Units in the amount of $0.65 per unit and a distribution on the 2012 OPP Units, 2013 MYLTIP Units and 2014 MYLTIP Units in the amount of $0.065 per unit, to holders of record as of the close of business on September 30, 2014. On December 8, 2014, the Company, as general partner of the Operating Partnership, declared a special cash distribution on the OP Units and LTIP Units in the amount of $4.50 per unit payable on January 28, 2015 to holders of record as of the close of business on December 31, 2014. The special cash distribution was in addition to the regular quarterly distribution on the OP Units and LTIP Units of $0.65 per unit and the distribution on the 2012 OPP Units, 2013 MYLTIP Units and 2014 MYLTIP Units in the amount of $0.065 per unit, in each case payable on January 28, 2015 to holders of record as of the close of business on December 31, 2014. Holders of the 2012 OPP Units, 2013 MYLTIP Units and 2014 MYLTIP Units are not entitled to receive any special distributions.
	A holder of an OP Unit may present such OP Unit to the Operating Partnership for redemption at any time (subject to restrictions agreed upon at the time of issuance of OP Units to particular holders that may restrict such redemption right for a period of time, generally one year from issuance). Upon presentation of an OP Unit for redemption, the Operating Partnership must redeem such OP Unit for cash equal to the then value of a share of common stock of the Company. The Company may, in its sole discretion, elect to assume and satisfy the redemption obligation by paying either cash or issuing one share of Common Stock. The value of the OP Units (not owned by the Company and including LTIP Units assuming that all conditions had been met for the conversion thereof) had all of such units been redeemed at December 31, 2014 was approximately $2.3 billion based on the closing price of the Company’s common stock of $128.69 per share on December 31, 2014.
	Noncontrolling Interests—Property Partnerships
	The noncontrolling interests in property partnerships consist of the outside equity interests in ventures that are consolidated with the financial results of the Company because the Company exercises control over the entities that own the properties. The equity interests in these ventures that are not owned by the Company, totaling approximately $1.6 billion at December 31, 2014 and approximately $726.1 million at December 31, 2013, are included in Noncontrolling Interests—Property Partnerships on the accompanying Consolidated Balance Sheets.
	On February 7, 2013, the partner in the Company's Salesforce Tower joint venture issued a notice that it was electing under the joint venture agreement to reduce its nominal ownership interest in the venture from 50% to 5%. On February 26, 2013, the Company issued a notice to the partner electing to proceed with the venture on that basis. As a result, the Company has a 95% nominal interest in and is consolidating the joint venture. Under the joint venture agreement, if certain return thresholds are achieved the partner will be entitled to an additional promoted interest. In addition, if the Company elects to fund the construction of Salesforce Tower without a construction loan (or a construction loan of less than 50% of project costs), then the partner has the option to require the Company to fund up to 2.5% of the total project costs (i.e., of 50% of the partner's 5% interest in the venture) in the form of a loan to the partner.  This loan would bear interest at the then prevailing market construction loan interest rates. Also, under the agreement, (1) the partner has the right to cause the Company to purchase the partner's interest after the defined stabilization date and (2) the Company has the right to acquire the partner's interest on the third anniversary of the stabilization date, in each case at an agreed upon purchase price or appraised value.
	On May 31, 2013, the Company's two joint venture partners in 767 Venture, LLC (the entity that owns 767 Fifth Avenue(the General Motors Building) in New York City) transferred all of their interests in the joint venture to third parties. In connection with the transfer, the Company and its new joint venture partners modified the Company's relative decision making authority and consent rights with respect to the joint venture's assets and operations. These changes resulted in the Company having sufficient financial and operating control over 767 Venture, LLC such that the Company now accounts for the assets, liabilities and operations of 767 Venture, LLC on a consolidated basis in its financial statements instead of under the equity method of accounting. Upon consolidation, the Company recognized the new joint venture partners' aggregate 40% equity interest at its aggregate fair value of approximately $480.9 million within Noncontrolling Interests—Property Partnerships on the accompanying Consolidated Balance Sheets.
	On October 9, 2013, the Company completed the sale of a 45% ownership interest in its Times Square Tower property for a gross sale price of $684.0 million in cash. Net cash proceeds totaled approximately $673.1 million, after the payment of transaction costs. In connection with the sale, the Company formed a limited liability company with the buyer and will provide customary property management and leasing services to the joint venture. Times Square Tower is an approximately 1,246,000 net rentable square foot Class A office tower located in New York City. The transaction did not qualify as a sale of real estate for financial reporting purposes because the Company effectively continues to control the property and thus will continue to account for the entity on a consolidated basis in its financial statements. The Company has accounted for the transaction as an equity transaction and has recognized noncontrolling interest in its consolidated balance sheets totaling approximately $243.5 million, which is equal to 45% of the carrying value of the total equity of the property immediately prior to the transaction. The difference between the net cash proceeds received and the noncontrolling interest recognized, which was approximately $429.6 million, has not been reflected as a gain on sale of real estate in the Company’s consolidated statements of operations and has instead been reflected as an increase to Additional Paid-in Capital in the Company’s Consolidated Balance Sheets.
	On October 30, 2014, the Company completed the sale of a 45% interest in each of 601 Lexington Avenue in New York City and Atlantic Wharf Office Building and 100 Federal Street in Boston for an aggregate gross sale price of approximately $1.827 billion in cash, less the partner’s pro rata share of the indebtedness collateralized by 601 Lexington Avenue. Net cash proceeds totaled approximately $1.497 billion, after the payment of transaction costs. In connection with the sale, the Company formed a limited liability company for each property with the buyer and will provide customary property management and leasing services to the joint ventures. 601 Lexington Avenue is a 1,669,000 square foot Class A office complex located in Midtown Manhattan. The property consists of a 59-story tower as well as a six-story low-rise office and retail building. The property is subject to existing mortgage indebtedness of approximately $712.9 million. The Atlantic Wharf Office Building is a 791,000 square foot Class A office tower located on Boston's Waterfront. 100 Federal Street is a 1,323,000 square foot Class A office tower located in Boston's Financial District. The transaction did not qualify as a sale of real estate for financial reporting purposes as the Company continues to effectively control these properties and thus will continue to account for the properties on a consolidated basis in its financial statements. The Company has accounted for the transaction as an equity transaction and has recognized noncontrolling interest in its consolidated balance sheets totaling approximately $849.0 million, which is equal to 45% of the aggregate carrying value of the total equity of the properties immediately prior to the transaction. The difference between the net cash proceeds received and the noncontrolling interest recognized, which was approximately $648.4 million, has not been reflected as a gain on sale of real estate in the Company's consolidated statements of operations and has instead been reflected as an increase in Additional Paid-in Capital in the Company's Consolidated Balance Sheets.

Stockholders_Equity

Stockholders' Equity	12 Months Ended
	Dec. 31, 2014
Equity [Abstract]
Stockholders' Equity	Stockholders’ Equity
	As of December 31, 2014, the Company had 153,113,945 shares of Common Stock outstanding.
	On June 3, 2014, the Company established a new “at the market” (ATM) stock offering program through which it may sell from time to time up to an aggregate of $600.0 million of its common stock through sales agents over a three-year period. This program replaced the Company’s prior $600.0 million ATM stock offering program that expired on June 2, 2014 with approximately $305.3 million of unsold common stock. The Company intends to use the net proceeds from any offering for general business purposes, which may include investment opportunities and debt reduction. No shares of common stock have been issued under this new ATM stock offering program.
	During the year ended December 31, 2014, the Company issued 21,459 shares of Common Stock upon the exercise of options to purchase Common Stock by certain employees. During the year ended December 31, 2013, the Company did not issued any shares of Common Stock upon the exercise of options to purchase Common Stock by employees.
	During the years ended December 31, 2014 and 2013, the Company issued 80,246 and 929,441 shares of Common Stock, respectively, in connection with the redemption of an equal number of OP Units.
	On January 29, 2014, the Company paid a special cash dividend and regular quarterly dividend aggregating $2.90 per share of Common Stock to shareholders of record as of the close of business on December 31, 2013. On April 30, 2014, the Company paid a dividend of $0.65 per share of Common Stock to shareholders of record as of the close of business on March 31, 2014. On July 31, 2014, the Company paid a dividend of $0.65 per share of Common Stock to shareholders of record as of the close of business on June 30, 2014. On October 31, 2014, the Company paid a dividend of $0.65 per share of Common Stock to shareholders of record as of the close of business on September 30, 2014.
	On December 8, 2014, the Company's Board of Directors declared a special cash dividend of $4.50 per share of Common Stock payable on January 28, 2015 to shareholders of record as of the close of business on December 31, 2014. The special cash dividend was in addition to the regular quarterly dividend of $0.65 per share of Common Stock declared by the Company's Board of Directors and payable on January 28, 2015 to shareholders of record as of the close of business on December 31, 2014.
	As of December 31, 2014, the Company had 80,000 shares (8,000,000 depositary shares each representing 1/100th of a share) outstanding of its 5.25% Series B Cumulative Redeemable Preferred Stock with a liquidation preference of $2,500.00 per share ($25.00 per depositary share). The Company pays cumulative cash dividends on the Series B Preferred Stock at a rate of 5.25% per annum of the $2,500.00 liquidation preference per share. The Company may not redeem the Series B Preferred Stock prior to March 27, 2018, except in certain circumstances relating to the preservation of the Company's REIT status. On or after March 27, 2018, the Company, at its option, may redeem the Series B Preferred Stock for a cash redemption price of $2,500.00 per share ($25.00 per depositary share), plus all accrued and unpaid dividends. The Series B Preferred Stock is not redeemable by the holders, has no maturity date and is not convertible into any other security of the Company or its affiliates.
	On February 18, 2014, the Company paid a dividend on its outstanding Series B Preferred Stock of $32.8125 per share. On May 15, 2014, the Company paid a dividend on its outstanding Series B Preferred Stock of $32.8125 per share. On August 15, 2014, the Company paid a dividend on its outstanding Series B Preferred Stock of $32.8125 per share. On November 17, 2014, the Company paid a dividend on its outstanding Series B Preferred Stock of $32.8125 per share. On December 8, 2014, the Company's Board of Directors declared a dividend of $32.8125 per share of Series B Preferred Stock payable on February 17, 2015 to shareholders of record as of the close of business on February 5, 2015.

Future_Minimum_Rents

Future Minimum Rents	12 Months Ended
	Dec. 31, 2014
Future Minimum Rents [Abstract]
Future Minimum Rents [Text Block]	13.    Future Minimum Rents
	The properties are leased to tenants under net operating leases with initial term expiration dates ranging from 2015 to 2046. The future contractual minimum lease payments to be received (excluding operating expense reimbursements) by the Company as of December 31, 2014, under non-cancelable operating leases which expire on various dates through 2046, are as follows:
	Years Ending December 31,	(in thousands)
	2015	$	1,796,517
	2016	1,788,978
	2017	1,677,005
	2018	1,568,200
	2019	1,483,188
	Thereafter	9,097,899
	No single tenant represented more than 10.0% of the Company’s total rental revenue for the years ended December 31, 2014, 2013 and 2012.

Segment_Information

Segment Information	12 Months Ended
	Dec. 31, 2014
Segment Reporting [Abstract]
Segment Information	Segment Information
	The Company’s segments are based on the Company’s method of internal reporting which classifies its operations by both geographic area and property type. The Company’s segments by geographic area are Boston, New York, San Francisco and Washington, DC. Segments by property type include: Class A Office, Office/Technical, Residential and Hotel.
	Asset information by segment is not reported because the Company does not use this measure to assess performance. Therefore, depreciation and amortization expense is not allocated among segments. Interest and other income, development and management services income, general and administrative expenses, transaction costs, impairment loss, interest expense, depreciation and amortization expense, gains from investments in securities, gains (losses) from early extinguishments of debt, income from unconsolidated joint ventures, gains on consolidation of joint ventures, discontinued operations, gains on sales of real estate and noncontrolling interests are not included in Net Operating Income as internal reporting addresses these items on a corporate level.
	Net Operating Income is not a measure of operating results or cash flows from operating activities as measured by accounting principles generally accepted in the United States of America, and it is not indicative of cash available to fund cash needs and should not be considered an alternative to cash flows as a measure of liquidity. All companies may not calculate Net Operating Income in the same manner. The Company considers Net Operating Income to be an appropriate supplemental measure to net income because it helps both investors and management to understand the core operations of the Company’s properties. The Company’s management also uses Net Operating Income to evaluate regional property level performance and to make decisions about resource allocations. Further, the Company believes Net Operating Income is useful to investors as a performance measure because, when compared across periods, Net Operating Income reflects the impact on operations from trends in occupancy rates, rental rates, operating costs and acquisition and development activity on an unleveraged basis, providing perspectives not immediately apparent from net income attributable to Boston Properties, Inc.
	On May 31, 2013, the Company's two joint venture partners in 767 Venture, LLC (the entity that owns 767 Fifth Avenue (the General Motors Building) located in New York City) transferred all of their interests in the joint venture to third parties (See Note 3). Effective as of May 31, 2013, the Company accounts for the assets, liabilities and operations of 767 Venture, LLC on a consolidated basis in its financial statements instead of under the equity method of accounting. Upon consolidation, the operations for this building are included in the New York region.
	Information by geographic area and property type (dollars in thousands):
	For the year ended December 31, 2014:
		Boston				New York			San				Washington,			Total
									Francisco				DC
	Rental Revenue:
	Class A Office	$	692,116			$	928,692		$	237,381			$	381,930		$	2,240,119
	Office/Technical	23,801				—			23,840				14,344			61,985
	Residential	4,528				—			—				21,665			26,193
	Hotel	43,385				—			—				—			43,385
	Total	763,830				928,692			261,221				417,939			2,371,682
	% of Grand Totals	32.21		%		39.16		%	11.01		%		17.62		%	100		%
	Rental Expenses:
	Class A Office	270,947				315,330			85,178				131,447			802,902
	Office/Technical	7,173				—			4,955				4,338			16,466
	Residential	1,957				—			—				13,965			15,922
	Hotel	29,236				—			—				—			29,236
	Total	309,313				315,330			90,133				149,750			864,526
	% of Grand Totals	35.78		%		36.47		%	10.43		%		17.32		%	100		%
	Net operating income	$	454,517			$	613,362		$	171,088			$	268,189		$	1,507,156
	% of Grand Totals	30.16		%		40.7		%	11.35		%		17.79		%	100		%
	For the year ended December 31, 2013:
		Boston				New York			San				Washington,			Total
									Francisco				DC
	Rental Revenue:
	Class A Office	$	665,991			$	725,566		$	214,755			$	381,359		$	1,987,671
	Office/Technical	22,617				—			17,259				15,649			55,525
	Residential	4,395				—			—				17,923			22,318
	Hotel	40,330				—			—				—			40,330
	Total	733,333				725,566			232,014				414,931			2,105,844
	% of Grand Totals	34.82		%		34.46		%	11.02		%		19.7		%	100		%
	Rental Expenses:
	Class A Office	259,997				251,640			77,905				126,507			716,049
	Office/Technical	6,879				—			3,708				4,190			14,777
	Residential	1,823				—			—				10,307			12,130
	Hotel	28,447				—			—				—			28,447
	Total	297,146				251,640			81,613				141,004			771,403
	% of Grand Totals	38.52		%		32.62		%	10.58		%		18.28		%	100		%
	Net operating income	$	436,187			$	473,926		$	150,401			$	273,927		$	1,334,441
	% of Grand Totals	32.69		%		35.51		%	11.27		%		20.53		%	100		%
	For the year ended December 31, 2012:
		Boston				New York			San				Washington,			Total
									Francisco				DC
	Rental Revenue:
	Class A Office	$	617,652			$	543,194		$	208,177			$	346,402		$	1,715,425
	Office/Technical	22,460				—			494				16,264			39,218
	Residential	3,936				—			—				16,632			20,568
	Hotel	37,915				—			—				—			37,915
	Total	681,963				543,194			208,671				379,298			1,813,126
	% of Grand Totals	37.61		%		29.96		%	11.51		%		20.92		%	100		%
	Rental Expenses:
	Class A Office	242,904				187,987			75,542				111,049			617,482
	Office/Technical	6,499				—			149				3,966			10,614
	Residential	1,675				—			—				9,317			10,992
	Hotel	28,120				—			—				—			28,120
	Total	279,198				187,987			75,691				124,332			667,208
	% of Grand Totals	41.85		%		28.18		%	11.34		%		18.63		%	100		%
	Net operating income	$	402,765			$	355,207		$	132,980			$	254,966		$	1,145,918
	% of Grand Totals	35.15		%		31		%	11.6		%		22.25		%	100		%
	The following is a reconciliation of Net Operating Income to net income attributable to Boston Properties, Inc. (in thousands):
	Year ended December 31,
		2014				2013			2012
Net Operating Income		$	1,507,156			$	1,334,441		$	1,145,918
Add:
Development and management services income		25,316				29,695			34,060
Income from unconsolidated joint ventures		12,769				75,074			49,078
Gains on consolidation of joint ventures		—				385,991			—
Interest and other income		8,765				8,310			10,091
Gains from investments in securities		1,038				2,911			1,389
Gains (losses) from early extinguishments of debt		(10,633		)		122			(4,453		)
Income from discontinued operations		—				8,022			9,806
Gains on sales of real estate from discontinued operations		—				112,829			36,877
Gain on forgiveness of debt from discontinued operations		—				20,182			—
Gains on sales of real estate		168,039				—			—
Less:
General and administrative expense		98,937				115,329			90,129
Transaction costs		3,140				1,744			3,653
Depreciation and amortization expense		628,573				560,637			445,875
Interest expense		455,743				446,880			410,970
Impairment loss		—				8,306			—
Impairment loss from discontinued operations		—				3,241			—
Noncontrolling interest in property partnerships		30,561				1,347			3,792
Noncontrolling interest—redeemable preferred units of the Operating Partnership		1,023				6,046			3,497
Noncontrolling interest—common units of the Operating Partnership		50,862				70,085			30,125
Noncontrolling interest in discontinued operations—common units of the Operating Partnership		—				14,151			5,075
Preferred dividends		10,500				8,057			—
Net income attributable to Boston Properties, Inc. common shareholders		$	433,111			$	741,754		$	289,650

Earnings_Per_Share

Earnings Per Share	12 Months Ended
	Dec. 31, 2014
Earnings Per Share [Abstract]
Earnings Per Share	Earnings Per Share
	The following table provides a reconciliation of both the net income attributable to Boston Properties, Inc. and the number of common shares used in the computation of basic earnings per share (“EPS”), which is calculated by dividing net income attributable to Boston Properties, Inc. by the weighted-average number of common shares outstanding during the period. The terms of the Series Two Preferred Units enable the holders to obtain OP Units of the Operating Partnership, as well as Common Stock of the Company, and as a result these are considered participating securities. Unvested share-based payment awards that contain non-forfeitable rights to dividends or dividend equivalents (whether paid or unpaid) are also participating securities. As such, unvested restricted common stock of the Company, LTIP Units, OPP Units and MYLTIP Units are considered participating securities. Participating securities are included in the computation of basic EPS of the Company using the two-class method. Participating securities are included in the computation of diluted EPS of the Company using the if-converted method if the impact is dilutive. Because the OPP Units and MYLTIP Units require the Company to outperform absolute and relative return thresholds, unless such thresholds have been met by the end of the applicable reporting period, the Company excludes such units from the diluted EPS calculation. Other potentially dilutive common shares, including stock options, restricted stock and other securities of the Operating Partnership that are exchangeable for the Company’s Common Stock, and the related impact on earnings, are considered when calculating diluted EPS.
		For the Year Ended December 31, 2014
		Income			Shares	Per Share
		(Numerator)			(Denominator)	Amount
		(in thousands, except for per share amounts)
	Basic Earnings:
	Net income attributable to Boston Properties, Inc. common shareholders	$	433,111		153,089	$	2.83
	Effect of Dilutive Securities:
	Stock Based Compensation	—			219	—
	Diluted Earnings:
	Net income attributable to Boston Properties, Inc. common shareholders	$	433,111		153,308	$	2.83
		For the Year Ended December 31, 2013
		Income			Shares	Per Share
		(Numerator)			(Denominator)	Amount
		(in thousands, except for per share amounts)
	Basic Earnings:
	Income from continuing operations attributable to Boston Properties, Inc.	$	618,113		152,201	$	4.06
	Discontinued operations attributable to Boston Properties, Inc.	123,641			—	0.81
	Allocation of undistributed earnings to participating securities	(160		)	—	—
	Net income attributable to Boston Properties, Inc. common shareholders	$	741,594		152,201	$	4.87
	Effect of Dilutive Securities:
	Stock Based Compensation and Exchangeable Senior Notes	—			320	(0.01		)
	Diluted Earnings:
	Net income attributable to Boston Properties, Inc. common shareholders	$	741,594		152,521	$	4.86
		For the Year Ended December 31, 2012
		Income			Shares	Per Share
		(Numerator)			(Denominator)	Amount
		(in thousands, except for per share amounts)
	Basic Earnings:
	Income from continuing operations attributable to Boston Properties, Inc.	$	248,042		150,120	$	1.65
	Discontinued operations attributable to Boston Properties, Inc.	41,608			—	0.28
	Net income attributable to Boston Properties, Inc. common shareholders	$	289,650		150,120	$	1.93
	Effect of Dilutive Securities:
	Stock Based Compensation and Exchangeable Senior Notes	—			591	(0.01		)
	Diluted Earnings:
	Net income attributable to Boston Properties, Inc. common shareholders	$	289,650		150,711	$	1.92

Employee_Benefit_Plans

Employee Benefit Plans	12 Months Ended
	Dec. 31, 2014
Employee Benefit Plans [Abstract]
Compensation and Employee Benefit Plans [Text Block]	16.    Employee Benefit Plans
	Effective January 1, 1985, the predecessor of the Company adopted a 401(k) Savings Plan (the “Plan”) for its employees. Under the Plan, as amended, employees, as defined, are eligible to participate in the Plan after they have completed three months of service. Upon formation, the Company adopted the Plan and the terms of the Plan.
	Under the Plan, as amended, the Company’s matching contribution equals 200% of the first 3% of participant’s eligible earnings contributed (utilizing earnings that are not in excess of an amount established by the IRS ($260,000, $255,000 and $250,000 in 2014, 2013 and 2012, respectively), indexed for inflation) with no vesting requirement. The Company’s aggregate matching contribution for the years ended December 31, 2014, 2013 and 2012 was $3.5 million, $3.4 million and $3.2 million, respectively.
	The Plan also provides for supplemental retirement contributions to certain employees who had at least ten years of service on January 1, 2001, and who were 40 years of age or older as of January 1, 2001. The maximum supplemental retirement contribution will not exceed the annual limit on contributions established by the IRS. The Company will record an annual supplemental retirement credit for the benefit of each participant. The Company’s supplemental retirement contribution and credit for the years ended December 31, 2014, 2013 and 2012 was $52,000, $60,000 and $78,000, respectively.
	The Company also maintains a deferred compensation plan that is designed to allow officers of the Company to defer a portion of their current income on a pre-tax basis and receive a tax-deferred return on these deferrals. The Company’s obligation under the plan is that of an unsecured promise to pay the deferred compensation to the plan participants in the future. At December 31, 2014 and 2013, the Company had maintained approximately $19.5 million and $16.6 million, respectively, in a separate account, which is not restricted as to its use. The Company’s liability under the plan is equal to the total amount of compensation deferred by the plan participants and earnings on the deferred compensation pursuant to investments elected by the plan participants. The Company’s liability as of December 31, 2014 and 2013 was $19.5 million and $16.6 million, respectively, which are included in the accompanying Consolidated Balance Sheets.

Stock_Option_and_Incentive_Pla

Stock Option and Incentive Plan	12 Months Ended
	Dec. 31, 2014
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Stock Option and Incentive Plan	Stock Option and Incentive Plan
	At the Company's 2012 annual meeting of stockholders held on May 15, 2012, the stockholders of the Company approved the Boston Properties, Inc. 2012 Stock Option and Incentive Plan (the “2012 Plan”). The 2012 Plan replaced the 1997 Stock Option and Incentive Plan (the “1997 Plan”). The material terms of the 2012 Plan include, among other things: (1) the maximum number of shares of common stock reserved and available for issuance under the 2012 Plan is the sum of (i) 13,000,000 newly authorized shares, plus (ii) the number of shares available for grant under the 1997 Stock Plan immediately prior to the effective date of the 2012 Plan, plus (iii) any shares underlying grants under the 1997 Plan that are forfeited, canceled or terminated (other than by exercise) in the future; (2) “full-value” awards (i.e., awards other than stock options) are multiplied by a 2.32 conversion ratio to calculate the number of shares available under the 2012 Plan that are used for each full-value award, as opposed to a 1.0 conversion ratio for each stock option awarded under the 2012 Plan; (3) shares tendered or held back for taxes will not be added back to the reserved pool under the 2012 Plan; (4) stock options may not be re-priced without stockholder approval; and (5) the term of the 2012 Plan is for ten years from the date of stockholder approval.
	On January 27, 2014, the Company's Compensation Committee approved the 2014 MYLTIP awards under the Company's 2012 Plan to certain officers and employees of the Company. The 2014 MYLTIP awards utilize TRS over a three-year measurement period, on an annualized, compounded basis, as the performance metric. Earned awards, if any, will be based on the Company's TRS relative to (i) the Cohen & Steers Realty Majors Portfolio Index (50% weight) and (ii) the NAREIT Office Index adjusted to exclude the Company (50% weight). Earned awards will range from $0 to a maximum of approximately $40.2 million depending on the Company's TRS relative to the two indices, with four tiers (threshold: approximately $6.7 million; target: approximately $13.4 million; high: approximately $26.8 million; and exceptional: approximately $40.2 million) and linear interpolation between tiers. Earned awards measured on the basis of relative TRS performance are subject to an absolute TRS component in the form of relatively simple modifiers that (A) reduce the level of earned awards in the event the Company's annualized TRS is less than 0% and (B) cause some awards to be earned in the event the Company's annualized TRS is more than 12% even though on a relative basis alone the Company's TRS would not result in any earned awards.
	Earned awards (if any) will vest 50% on February 3, 2017 and 50% on February 3, 2018, based on continued employment. Vesting will be accelerated in the event of a change in control, termination of employment by the Company without cause, termination of employment by the award recipient for good reason, death, disability or retirement. If there is a change of control prior to February 3, 2017, earned awards will be calculated as of the date of the change of control based upon performance through such date as measured against performance hurdles (without proration). The 2014 MYLTIP awards are in the form of LTIP Units issued on the grant date which (i) are subject to forfeiture to the extent awards are not earned and (ii) prior to the performance measurement date are only entitled to one-tenth (10%) of the regular quarterly distributions payable on common partnership units.
	Under the FASB's ASC 718 “Compensation-Stock Compensation” the 2014 MYLTIP awards have an aggregate value of approximately $12.7 million, which amount will generally be amortized into earnings over the four-year plan period under the graded vesting method.
	On January 31, 2014, the measurement period for the Company’s 2011 OPP Unit awards expired and the Company’s TRS was not sufficient for employees to earn and therefore become eligible to vest in any of the 2011 OPP Unit awards. As a result, the Company accelerated the then remaining unrecognized compensation expense totaling approximately $1.2 million during the year ended December 31, 2014. Accordingly, all 2011 OPP Unit awards were automatically forfeited.
	On March 11, 2013, the Company announced that Owen D. Thomas would succeed Mortimer B. Zuckerman as the Company's Chief Executive Officer, effective April 2, 2013. On April 2, 2013, the Company issued 24,231 LTIP units, 38,926 2013 MYLTIP Units and 50,847 non-qualified stock options under the 2012 Plan to Mr. Thomas, pursuant to his employment agreement. Mr. Zuckerman continued to serve as Executive Chairman for a transition period which was completed effective as of the close of business on December 31, 2014 and thereafter is continuing to serve as the non-executive Chairman of the Board.  In connection with succession planning, the Company and Mr. Zuckerman entered into a Transition Benefits Agreement. If Mr. Zuckerman remained employed by the Company through July 1, 2014, he was entitled to receive on January 1, 2015 a lump sum cash payment of $6.7 million and an equity award with a targeted value of approximately $11.1 million. The cash payment and equity award vested one-third on each of March 10, 2013, October 1, 2013 and July 1, 2014. As a result, the Company recognized approximately $3.9 million and $13.8 million of compensation expense during the years ended December 31, 2014 and 2013, respectively. In addition, the agreement provided that if Mr. Zuckerman terminated his employment with the Company for any reason, voluntarily or involuntarily, he would become fully vested in any outstanding equity awards with time-based vesting. As a result, during the year ended December 31, 2013, the Company accelerated the remaining approximately $12.9 million of stock-based compensation expense associated with Mr. Zuckerman's unvested long-term equity awards.
	The Company issued 23,968, 36,730 and 20,756 shares of restricted common stock and 127,094, 184,733 and 174,650 LTIP Units to employees and non-employee directors under the 1997 Plan and 2012 Plan during the years ended December 31, 2014, 2013 and 2012, respectively. The Company did not issue any non-qualified stock options under the 1997 Plan during the year ended December 31, 2014. The Company issued 252,220 and 186,007 non-qualified stock options under the 1997 Plan during the years ended December 31, 2013 and 2012, respectively. The amounts issued during 2013 include the amounts issued to Mr. Thomas pursuant to his employment agreement, as discussed above. The Company issued 400,000 2012 OPP Units to employees under the 1997 Plan during the year ended December 31, 2012. The Company issued 318,926 2013 MYLTIP Units to employees under the 2012 Plan during the year ended December 31, 2013, including the amounts issued to Mr. Thomas pursuant to his employment agreement, as discussed above. The Company issued 485,459 2014 MYLTIP Units to employees under the 2012 Plan during the year ended December 31, 2014. Employees and directors paid $0.01 per share of restricted common stock and $0.25 per LTIP Unit, OPP Unit and MYLTIP Unit. An LTIP Unit is generally the economic equivalent of a share of restricted stock in the Company. The aggregate value of the LTIP Units is included in noncontrolling interests in the Consolidated Balance Sheets. Grants of restricted stock and LTIP Units to employees vest in four equal annual installments. Restricted stock is measured at fair value on the date of grant based on the number of shares granted, as adjusted for forfeitures, and the closing price of the Company’s Common Stock on the date of grant as quoted on the New York Stock Exchange. Such value is recognized as an expense ratably over the corresponding employee service period. Non-qualified stock options, which are valued using the Black-Scholes option-pricing model, are recognized as an expense ratably over the corresponding employee service period. As the 2012 OPP Awards, 2013 MYLTIP Awards and 2014 MYLTIP Awards are subject to both a service condition and a market condition, the Company recognizes the compensation expense related to the 2012 OPP Awards, 2013 MYLTIP Awards and 2014 MYLTIP Awards under the graded vesting attribution method. Under the graded vesting attribution method, each portion of the award that vests at a different date is accounted for as a separate award and recognized over the period appropriate to that portion so that the compensation cost for each portion should be recognized in full by the time that portion vests. Dividends paid on both vested and unvested shares of restricted stock are charged directly to Dividends in Excess of Earnings in the Consolidated Balance Sheets. Aggregate stock-based compensation expense associated with restricted stock, non-qualified stock options, LTIP Units, 2011 OPP Units, 2012 OPP Units, 2013 MYLTIP Units and 2014 MYLTIP Units was approximately $26.0 million, $43.9 million and $28.3 million for the years ended December 31, 2014, 2013 and 2012, respectively. For the years ended December 31, 2014 and 2013, stock-based compensation expense includes approximately $2.5 million and $21.5 million, respectively, consisting of the acceleration of the expense of the Company’s Executive Chairman’s stock-based compensation awards and the stock-based compensation awards associated with his transition benefits agreement related to the Company's succession planning. For the year ended December 31, 2012, stock-based compensation expense includes approximately $2.7 million consisting of the acceleration of vesting of the Company’s Chief Operating Officer’s stock-based compensation awards associated with his resignation. At December 31, 2014, there was $17.0 million of unrecognized compensation expense related to unvested restricted stock and LTIP Units and $14.5 million of unrecognized compensation expense related to unvested 2012 OPP Units, 2013 MYLTIP Units and 2014 MYLTIP Units that is expected to be recognized over a weighted-average period of approximately 2.7 years.
	The shares of restricted stock were valued at approximately $2.6 million ($109.27 per share weighted-average), $3.9 million ($105.30 per share weighted-average) and $2.2 million ($107.31 per share weighted-average) for the years ended December 31, 2014, 2013 and 2012, respectively.
	LTIP Units were valued using a Monte Carlo simulation method model in accordance with the provisions of ASC 718 “Compensation—Stock Compensation” (“ASC 718”). LTIP Units issued during the years ended December 31, 2014, 2013 and 2012 were valued at approximately $12.8 million, $17.8 million and $17.3 million, respectively. The weighted-average per unit fair value of LTIP Unit grants in 2014, 2013 and 2012 was $100.61, $96.13 and $98.83, respectively. The per unit fair value of each LTIP Unit granted in 2014, 2013 and 2012 was estimated on the date of grant using the following assumptions; an expected life of 5.7 years, 5.7 years and 5.8 years, a risk-free interest rate of 1.84%, 1.03% and 0.94% and an expected price volatility of 27.0%, 26.0% and 29.1%, respectively.
	There were no non-qualified stock options granted during the year ended December 31, 2014. The non-qualified stock options granted during the years ended December 31, 2013 and 2012 had a weighted-average fair value on the date of grant of $18.46 and $19.50 per option, respectively, which was computed using the Black-Scholes option-pricing model utilizing the following assumptions: an expected life of 6.0 years and 5.4 years, a risk-free interest rate of 1.11% and 0.92%, an expected price volatility of 26.0% and 28.4% and an expected dividend yield of 3.0% and 2.9%, respectively. The exercise price of the options granted during the years ended December 31, 2013 and 2012 were $105.10 and $107.23, respectively, which was the closing price of the Company’s common stock on the date of grant.
	A summary of the status of the Company’s stock options as of December 31, 2014, 2013 and 2012 and changes during the years then ended are presented below:
		Shares		Weighted
				Average
				Exercise
				Price
	Outstanding at December 31, 2011	155,623		$	89.35
	Granted	186,007		$	107.23
	Exercised	(22,823	)	$	72.42
	Canceled	(24,280	)	$	100.15
	Outstanding at December 31, 2012	294,527		$	101.06
	Granted	252,220		$	104.5
	Special dividend adjustment	12,076		$	100.44
	Outstanding at December 31, 2013	558,823		$	100.43
	Exercised	(21,459	)	$	97.04
	Canceled	(2,444	)	$	103.57
	Special dividend adjustment	18,392		$	97.22
	Outstanding at December 31, 2014	553,312		$	97.21
	The following table summarizes information about stock options outstanding at December 31, 2014:
	Options Outstanding					Options Exercisable
	Number		Weighted-Average Remaining	Exercise Price		Number Exercisable	Exercise Price
	Outstanding at		Contractual Life			at 12/31/14
	12/31/14
	124,513		3.6 years	$	87.7	111,832	$	87.7
	53,759		8.3 years	$	96.62	13,439	$	96.62
	207,064		4.9 years	$	99.41	150,024	$	99.41
	167,976		4.4 years	$	101.75	135,848	$	101.75
	The total intrinsic value of the outstanding and exercisable stock options as of December 31, 2014 was approximately $13.1 million. In addition, the Company had 199,868 and 91,496 options exercisable at a weighted-average exercise price of $98.83 and $98.92 at December 31, 2013 and 2012, respectively.
	The Company adopted the 1999 Non-Qualified Employee Stock Purchase Plan (the “Stock Purchase Plan”) to encourage the ownership of Common Stock by eligible employees. The Stock Purchase Plan became effective on January 1, 1999 with an aggregate maximum of 250,000 shares of Common Stock available for issuance. The Stock Purchase Plan provides for eligible employees to purchase on the business day immediately following the end of the biannual purchase periods (i.e., January 1-June 30 and July 1-December 31) shares of Common Stock at a purchase price equal to 85% of the average closing prices of the Common Stock during the last ten business days of the purchase period. The Company issued 6,964, 6,442 and 7,406 shares with the weighted average purchase price equal to $93.37 per share, $89.65 per share and $86.52 per share under the Stock Purchase Plan during the years ended December 31, 2014, 2013 and 2012, respectively.

Related_Party_Transactions

Related Party Transactions	12 Months Ended
	Dec. 31, 2014
Related Party Transaction [Line Items]
Related Party Transactions Disclosure [Text Block]	18.    Related Party Transactions
	Prior to joining the Company effective January 2, 2014, Mr. John F. Powers provided commercial real estate brokerage services to the Company, on behalf of his prior employer, CBRE, in connection with certain leasing transactions. Mr. Powers received approximately $1,214,000 and $592,000 during the years ended December 31, 2014 and 2013, respectively, in connection with these transactions. Mr. John F. Powers is a Senior Vice President of Boston Properties, Inc. and the Regional Manager of its New York office.
	A firm controlled by Mr. Raymond A. Ritchey’s brother was paid aggregate leasing commissions of approximately $674,000, $868,000 and $1,306,000 for the years ended December 31, 2014, 2013 and 2012, respectively, related to certain exclusive leasing arrangements for certain Northern Virginia properties. Mr. Ritchey is an Executive Vice President of Boston Properties, Inc.
	On June 30, 1998, the Company acquired from entities controlled by Mr. Alan B. Landis a portfolio of properties known as the Carnegie Center Portfolio and Tower Center One and related operations and development rights (collectively, the “Carnegie Center Portfolio”). Mr. A. Landis is the brother of Mr. Mitchell S. Landis, the Company's former Senior Vice President and Regional Manager of the Company's Princeton office. Mr. M. Landis' employment with the Company terminated on March 31, 2014. In connection with the acquisition of the Carnegie Center Portfolio, the Operating Partnership entered into a development agreement (the “Development Agreement”) with affiliates of Mr. A. Landis providing for up to approximately 2,000,000 square feet of development in or adjacent to the Carnegie Center office complex. An affiliate of Mr. A. Landis was entitled to a purchase price for each parcel developed under the Development Agreement calculated on the basis of $20 per rentable square foot of property developed. Another affiliate of Mr. A. Landis was eligible to earn a contingent payment for each developed property that achieves a stabilized return in excess of a target annual return ranging between 10.5% and 11%. The Development Agreement also provided that upon negotiated terms and conditions, the Company and Mr. A. Landis would form a development company to provide development services for these development projects and would share the expenses and profits, if any, of this new company.
	On October 21, 2004, the Company and Mr. A. Landis entered into an agreement (the “2004 Agreement”) to modify several provisions of the Development Agreement. Under the terms of the 2004 Agreement, the Operating Partnership and affiliates of Mr. A. Landis amended the Development Agreement to limit the rights of Mr. A. Landis and his affiliates to participate in the development of properties under the Development Agreement. Among other things, Mr. A. Landis agreed that (1) Mr. A. Landis and his affiliates will have no right to participate in any entity formed to acquire land parcels or the development company formed by the Operating Partnership to provide development services under the Development Agreement, (2) Mr. A. Landis will have no right or obligation to play a role in development activities engaged in by the development company formed by the Operating Partnership under the Development Agreement or receive compensation from the development company and (3) the affiliate of Mr. A. Landis will have no right to receive a contingent payment for developed properties based on stabilized returns. In exchange, the Company (together with the Operating Partnership) agreed to:
	•	effective as of June 30, 1998, pay Mr. A. Landis $125,000 on January 1 of each year until the earlier of (A) January 1, 2018, (B) the termination of the Development Agreement or (C) the date on which all development properties under the Development Agreement have been conveyed pursuant to the Development Agreement, with $750,000, representing payments of this annual amount from 1998 to 2004, being paid upon execution of the 2004 Agreement; and
	•	pay an affiliate of Mr. A. Landis, in connection with the development of land parcels acquired under the Development Agreement, an aggregate fixed amount of $10.50 per rentable square foot of property developed (with a portion of this amount (i.e., $5.50) being subject to adjustment, in specified circumstances, based on future increases in the Consumer Price Index) in lieu of a contingent payment based on stabilized returns, which payment could have been greater or less than $10.50 per rentable square foot of property developed.
	The Operating Partnership also continues to be obligated to pay an affiliate of Mr. A. Landis the purchase price of $20 per rentable square foot of property developed for each land parcel acquired as provided in the original Development Agreement. During the 20-year term of the Development Agreement, until such time, if any, as the Operating Partnership elects to acquire a land parcel, an affiliate of Mr. A. Landis will remain responsible for all carrying costs associated with such land parcel. Pursuant to the Development Agreement, as amended by the 2004 Agreement, the Company paid Mr. A. Landis $125,000 on each of January 1, 2013 and January 1, 2014. On November 12, 2014, the Company acquired from Mr. A. Landis 804 Carnegie Center, a land parcel located in Princeton, New Jersey for a purchase price of approximately $3.7 million.
	In accordance with the Company’s 2012 Plan, and as approved by the Board of Directors, six non-employee directors made an election to receive deferred stock units in lieu of cash fees for 2014. The deferred stock units will be settled in shares of common stock upon the cessation of such director’s service on the Board of Directors. As a result of these elections, the aggregate cash fees otherwise payable to a non-employee director during a fiscal quarter are converted into a number of deferred stock units equal to the aggregate cash fees divided by the last reported sales price of a share of the Company’s common stock on the last trading of the applicable fiscal quarter. The deferred stock units are also credited with dividend equivalents as dividends are paid by the Company. On May 20, 2014, in connection with the cessation of a director's service on the Board of Directors, the Company issued 7,542 shares of common stock in settlement of the director's outstanding deferred stock units. At December 31, 2014 and 2013, the Company had outstanding 84,435 and 83,995 deferred stock units, respectively.

Selected_Interim_Financial_Inf

Selected Interim Financial Information (unaudited)	12 Months Ended
	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Quarterly Financial Information [Text Block]	9.    Selected Interim Financial Information (unaudited)
	The tables below reflect the Company’s selected quarterly information for the years ended December 31, 2014 and 2013.
		2014 Quarter Ended
		March 31,		June 30,		September 30,		December 31,
		(in thousands, except for per share amounts)
	Total revenue	$	574,694	$	589,794	$	618,803	$	613,707
	Income from continuing operations	$	67,756	$	95,901	$	109,038	$	85,323
	Net income attributable to Boston Properties, Inc. common sharedholders	$	54,034	$	76,527	$	127,724	$	174,510
	Income attributable to Boston Properties, Inc. per share—basic	$	0.35	$	0.5	$	0.83	$	1.14
	Income attributable to Boston Properties, Inc. per share—diluted	$	0.35	$	0.5	$	0.83	$	1.14
		2013 Quarter Ended
		March 31,		June 30,		September 30,		December 31,
		(in thousands, except for per share amounts)
	Total revenue	$	477,826	$	510,033	$	571,481	$	576,199
	Income from continuing operations	$	38,496	$	503,446	$	82,311	$	79,395
	Net income attributable to Boston Properties, Inc. common sharedholders	$	47,854	$	452,417	$	152,677	$	88,719
	Income attributable to Boston Properties, Inc. per share—basic	$	0.32	$	2.95	$	1	$	0.58
	Income attributable to Boston Properties, Inc. per share—diluted	$	0.31	$	2.94	$	1	$	0.58

Subsequent_Events

Subsequent Events	12 Months Ended
	Dec. 31, 2014
Subsequent Events [Abstract]
Subsequent Events	Subsequent Events
	On January 15, 2015, the Company entered into a contract for the sale of its Residences on The Avenue property located in Washington, DC for a gross sale price of $196.0 million. The Company has agreed to provide net operating income support of up to $6.0 million should the property’s net operating income fail to achieve certain thresholds. The Residences on The Avenue is comprised of 335 apartment units and approximately 50,000 net rentable square feet of retail space, subject to a ground lease that expires on February 1, 2068. The sale is subject to the satisfaction of customary closing conditions and there can be no assurance that the sale will be consummated on the terms currently contemplated or at all.
	On January 21, 2015, the Company's Compensation Committee approved the 2015 Multi-Year, Long-Term Incentive Program (the "2015 MYLTIP") awards under the Company's 2012 Plan to certain officers and employees of the Company. The 2015 MYLTIP awards utilize TRS over a three-year measurement period, on an annualized, compounded basis, as the performance metric. Earned awards will be based on the Company's TRS relative to (i) the Cohen & Steers Realty Majors Portfolio Index (50% weight) and (ii) the NAREIT Office Index adjusted to exclude the Company (50% weight). Earned awards will range from zero to a maximum of approximately $40.8 million depending on the Company's TRS relative to the two indices, with three tiers (threshold: approximately $8.2 million; target: approximately $16.3 million; high: approximately $40.8 million) and linear interpolation between tiers. Earned awards measured on the basis of relative TRS performance are subject to an absolute TRS component in the form of relatively simple modifiers that (A) reduce the level of earned awards in the event the Company's annualized TRS is less than 0% and (B) cause some awards to be earned in the event the Company's annualized TRS is more than 12% even though on a relative basis alone the Company's TRS would not result in any earned awards.
	Earned awards (if any) will vest 50% on February 4, 2018 and 50% on February 4, 2019, based on continued employment. Vesting will be accelerated in the event of a change in control, termination of employment by the Company without cause, or termination of employment by the award recipient for good reason, death, disability or retirement. If there is a change of control prior to February 4, 2018, earned awards will be calculated based on TRS performance up to the date of the change of control. The 2015 MYLTIP awards are in the form of LTIP Units issued on the grant date which (i) are subject to forfeiture to the extent awards are not earned and (ii) prior to the performance measurement date are only entitled to one-tenth (10%) of the regular quarterly distributions payable on common partnership units.
	Under the Financial Accounting Standards Board's Accounting Standards Codification (“ASC”) 718 “Compensation-Stock Compensation” the 2015 MYLTIP awards have an aggregate value of approximately $15.7 million, which amount will generally be amortized into earnings over the four-year plan period under the graded vesting method.
	On February 3, 2015, the Company issued 30,965 shares of restricted common stock and 96,830 LTIP units under the 2012 Plan to certain employees of the Company.
	On February 6, 2015, the measurement period for the Company’s 2012 OPP Awards ended and the Company’s TRS performance was sufficient for employees to earn and therefore become eligible to vest in the 2012 OPP Awards. The final outperformance pool was determined to be approximately $32.1 million, or approximately 80% of the total maximum outperformance pool of $40.0 million.
	On February 19, 2015, the Company completed the sale of a parcel of land within its Washingtonian North property located in Gaithersburg, Maryland for a sale price of approximately $8.7 million, which exceeded its carrying value. The parcel contains approximately 8.5 acres of the approximately 27 acre property.
	During February 2015, the Company entered into forward-starting interest rate swap contracts which fix the ten-year swap rate at a weighted-average rate of 2.514% per annum on notional amounts aggregating $125.0 million. The interest rate swap contracts were entered into in advance of a financing with a target commencement date in September 2016 and expiration in September 2026.

Real_Estate_and_Accumulated_De

Real Estate and Accumulated Depreciation	12 Months Ended
	Dec. 31, 2014
SEC Schedule III, Real Estate and Accumulated Depreciation Disclosure [Abstract]
SEC Schedule III, Real Estate and Accumulated Depreciation Disclosure [Text Block]	Boston Properties, Inc.
	Schedule 3 - Real Estate and Accumulated Depreciation
	December 31, 2014
	(dollars in thousands)
	Property Name		Type		Location	Encumbrances					Original				Costs			Land and Improvements		Building and		Land		Development		Total		Accumulated		Year(s) Built/	Depreciable
															Capitalized					Improvements		Held for		and				Depreciation		Renovated	Lives  (Years)
															Subsequent							Development		Construction
															to									in Progress
											Land		Building		Acquisition
	767 Fifth Avenue (The General Motors Building)		Office		New York, NY	$	1,421,083				$	1,796,252	$	1,532,654	$	19,627		$	1,796,252	$	1,552,281	$	—	$	—	$	3,348,533	$	80,075	1968	-1
	Embarcadero Center		Office		San Francisco, CA	354,680					179,697		847,410		317,320			195,987		1,148,440		—		—		1,344,427		505,702		1970/1989	-1
	Prudential Center		Office		Boston, MA	—					92,077		734,594		380,216			107,426		1,064,013		404		35,044		1,206,887		421,371		1965/1993/2002	-1
	399 Park Avenue		Office		New York, NY	—					339,200		700,358		109,630			354,107		795,081		—		—		1,149,188		242,572		1961	-1
	601 Lexington Avenue		Office		New York, NY	710,932					241,600		494,782		230,137			289,639		676,880		—		—		966,519		228,397		1977/1997	-1
	The John Hancock Tower and Garage		Office		Boston, MA	649,108					219,543		667,884		74,060			219,616		740,863		1,008		—		961,487		104,888		1976	-1
	250 West 55th Street		Office		New York, NY	—					285,263		603,167		—			285,263		603,167		—		—		888,430		9,240		2014	-1
	Times Square Tower		Office		New York, NY	—					165,413		380,438		84,488			169,193		461,146		—		—		630,339		150,619		2004	-1
	100 Federal Street		Office		Boston, MA	—					131,067		435,954		9,711			131,067		445,432		233		—		576,732		44,931		1971-1975	-1
	Carnegie Center		Office		Princeton, NJ	—					105,107		377,259		83,053			103,064		450,653		2,350		9,352		565,419		183,866		1983-1999	-1
	Atlantic Wharf		Office		Boston, MA	—					63,988		454,537		15,169			63,988		469,706		—		—		533,694		55,070		2011	-1
	Fountain Square		Office		Reston, VA	220,133					56,853		306,298		6,938			56,853		313,236		—		—		370,089		27,645		1986-1990	-1
	510 Madison Avenue		Office		New York, NY	—					103,000		253,665		12,912			103,000		266,577		—		—		369,577		24,871		2012	-1
	599 Lexington Avenue		Office		New York, NY	750,000					81,040		100,507		128,649			87,852		222,344		—		—		310,196		152,958		1986	-1
	680 Folsom Street		Office		San Francisco, CA	—					106,510		185,801		—			106,510		185,801		—		—		292,311		4,161		2014	-1
	South of Market and Democracy Tower		Office		Reston, VA	—					13,603		237,479		9,790			13,687		247,185		—		—		260,872		60,668		2008-2009	-1
	Bay Colony Corporate Center		Office		Waltham, MA	—					18,789		148,451		58,126			18,789		206,577		—		—		225,366		31,089		1985-1989	-1
	Gateway Center		Office		San Francisco, CA	—					28,255		139,245		52,768			30,627		189,641		—		—		220,268		87,759		1984/1986/2002	-1
	2200 Pennsylvania Avenue		Office		Washington, DC	—					—		183,541		5,141			—		188,682		—		—		188,682		26,612		2011	-1
	Mountain View Research Park		Office		Mountain View, CA	—					95,066		68,373		3,267			95,066		71,640		—		—		166,706		6,044		1977-1981/2007-2013	-1
	3100-3130 Zanker Road (formerly 3200 Zanker Road)		Office		San Jose, CA	—					36,705		82,863		28,919			36,997		107,737		3,753		—		148,487		25,914		1988	-1
	Reservoir Place		Office		Waltham, MA	—					18,605		92,619		34,732			20,118		125,830		8		—		145,956		57,456		1955/1987	-1
	1333 New Hampshire Avenue		Office		Washington, DC	—					34,032		85,660		9,947			35,382		94,257		—		—		129,639		32,942		1996	-1
	505 9th Street		Office		Washington, DC	118,919					38,885		83,719		5,692			42,082		86,214		—		—		128,296		23,366		2007	-1
	Kingstowne Towne Center		Office		Alexandria, VA	31,364					18,021		109,038		1,013			18,062		110,010		—		—		128,072		29,821		2003-2006	-1
	1330 Connecticut Avenue		Office		Washington, DC	—					25,982		82,311		16,379			27,135		97,537		—		—		124,672		32,742		1984	-1
	Capital Gallery		Office		Washington, DC	—					4,725		29,565		88,078			8,662		113,706		—		—		122,368		53,320		1981/2006	-1
	Weston Corporate Center		Office		Weston, MA	—					25,753		92,312		(123		)	25,854		92,088		—		—		117,942		13,972		2010	-1
	One Freedom Square		Office		Reston, VA	—					9,929		84,504		18,898			11,293		102,038		—		—		113,331		36,912		2000	-1
	Two Freedom Square		Office		Reston, VA	—					13,930		77,739		19,563			15,420		95,812		—		—		111,232		38,275		2001	-1
	One and Two Reston Overlook		Office		Reston, VA	—					16,456		66,192		25,320			16,179		91,789		—		—		107,968		35,274		1999	-1
	415 Main Street (formerly Seven Cambridge Center)		Office		Cambridge, MA	—					3,457		97,136		2,880			4,125		99,348		—		—		103,473		53,469		2006	-1
	Discovery Square		Office		Reston, VA	—					11,198		71,782		19,977			12,533		90,424		—		—		102,957		32,326		2001	-1
	140 Kendrick Street		Office		Needham, MA	—					18,095		66,905		14,870			19,092		80,778		—		—		99,870		20,193		2000	-1
	355 Main Street (formerly Five Cambridge Center)		Office		Cambridge, MA	—					18,863		53,346		25,899			21,173		76,935		—		—		98,108		20,864		1981/1996/2013	-1
	90 Broadway (formerly Four Cambridge Center)		Office		Cambridge, MA	—					19,104		52,078		13,680			20,785		64,077		—		—		84,862		12,426		1983/1998/2013	-1
	Waltham Weston Corporate Center		Office		Waltham, MA	—					10,385		60,694		8,105			11,097		68,087		—		—		79,184		22,151		2003	-1
	230 CityPoint		Office		Waltham, MA	—					13,189		49,823		15,744			13,593		65,163		—		—		78,756		19,029		1992	-1
	77 CityPoint		Office		Waltham, MA	—					13,847		60,383		3,163			13,873		63,520		—		—		77,393		14,639		2008	-1
	North First Business Park		Office		San Jose, CA	—					58,402		13,069		4,028			23,377		16,214		35,908		—		75,499		12,174		1981	-1
	300 Binney Street (formerly Seventeen Cambridge Center)		Office		Cambridge, MA	—					18,080		51,262		165			18,080		51,427		—		—		69,507		2,938		2013	-1
	2440 West El Camino Real		Office		Mountain View, CA	—					16,741		51,285		716			16,741		52,001		—		—		68,742		6,400		1987/2003	-1
	Wisconsin Place		Office		Chevy Chase, MD	—					—		53,349		12,787			—		66,136		—		—		66,136		14,300		2009	-1
	Reston Corporate Center		Office		Reston, VA	—					9,135		50,857		5,361			10,148		55,205		—		—		65,353		22,540		1984	-1
	New Dominion Technology Park, Bldg. Two		Office		Herndon, VA	—					5,584		51,868		3,911			6,510		54,853		—		—		61,363		18,312		2004	-1
	Sumner Square		Office		Washington, DC	—					624		28,745		25,004			1,478		52,895		—		—		54,373		25,152		1985	-1
	New Dominion Technology Park, Bldg. One		Office		Herndon, VA	40,975					3,880		43,227		3,883			4,583		46,407		—		—		50,990		21,158		2001	-1
	200 West Street		Office		Waltham, MA	—					16,148		24,983		9,782			16,813		34,100		—		—		50,913		16,439		1999	-1
	191 Spring Street		Office		Lexington, MA	—					2,850		27,166		18,558			3,151		45,423		—		—		48,574		31,087		1971/1995	-1
	255 Main Street (formerly One Cambridge Center)		Office		Cambridge, MA	—					134		25,110		20,596			548		45,292		—		—		45,840		27,168		1987	-1
	University Place		Office		Cambridge, MA	12,290					—		37,091		6,794			390		43,495		—		—		43,885		22,156		1985	-1
	2600 Tower Oaks Boulevard		Office		Rockville, MD	—					4,243		31,125		7,924			4,785		38,507		—		—		43,292		16,993		2001	-1
	Quorum Office Park		Office		Chelmsford, MA	—					3,750		32,454		5,814			5,187		36,831		—		—		42,018		13,710		2001	-1
	500 E Street		Office		Washington, DC	—					109		22,420		13,020			2,379		33,170		—		—		35,549		21,601		1987	-1
	150 Broadway (formerly Eight Cambridge Center)		Office		Cambridge, MA	—					850		25,042		8,507			1,323		33,076		—		—		34,399		12,606		1999	-1
325 Main Street (formerly Three Cambridge Center)		Office		Cambridge, MA	—					174		12,200		11,774			965		23,183		—		—		24,148		10,496		1987/2013	-1
105 Broadway (formerly Ten Cambridge Center)		Office		Cambridge, MA	—					1,299		12,943		6,851			2,395		18,698		—		—		21,093		11,948		1990	-1
40 Shattuck Road		Office		Andover, MA	—					709		14,740		2,697			893		17,253		—		—		18,146		6,171		2001	-1
201 Spring Street		Office		Lexington, MA	—					2,849		15,303		(154		)	3,124		14,874		—		—		17,998		6,511		1997	-1
Lexington Office Park		Office		Lexington, MA	—					998		1,426		13,941			1,264		15,101		—		—		16,365		11,346		1982	-1
92-100 Hayden Avenue		Office		Lexington, MA	—					594		6,748		7,819			802		14,359		—		—		15,161		10,665		1985	-1
91 Hartwell Avenue		Office		Lexington, MA	—					784		6,464		6,747			941		13,054		—		—		13,995		7,655		1985	-1
181 Spring Street		Office		Lexington, MA	—					1,066		9,520		1,950			1,160		11,376		—		—		12,536		4,306		1999	-1
195 West Street		Office		Waltham, MA	—					1,611		6,652		4,267			1,858		10,672		—		—		12,530		6,626		1990	-1
33 Hayden Avenue		Office		Lexington, MA	—					266		3,234		8,787			425		11,862		—		—		12,287		5,558		1979	-1
145 Broadway (formerly Eleven Cambridge Center)		Office		Cambridge, MA	—					121		5,535		5,278			324		10,610		—		—		10,934		7,566		1984	-1
7501 Boston Boulevard, Building Seven		Office		Springfield, VA	—					665		9,273		520			791		9,667		—		—		10,458		4,113		1997	-1
7435 Boston Boulevard, Building One		Office		Springfield, VA	—					392		3,822		3,798			659		7,353		—		—		8,012		5,794		1982	-1
7450 Boston Boulevard, Building Three		Office		Springfield, VA	—					1,165		4,681		1,880			1,430		6,296		—		—		7,726		3,056		1987	-1
8000 Grainger Court, Building Five		Office		Springfield, VA	—					366		4,282		2,684			601		6,731		—		—		7,332		5,142		1984	-1
453 Ravendale Drive		Office		Mountain View, CA	—					5,477		1,090		230			5,477		1,320		—		—		6,797		213		1977	-1
7500 Boston Boulevard, Building Six		Office		Springfield, VA	—					138		3,749		2,538			406		6,019		—		—		6,425		4,265		1985	-1
7601 Boston Boulevard, Building Eight		Office		Springfield, VA	—					200		878		4,826			551		5,353		—		—		5,904		3,688		1986	-1
250 Binney Street (formerly Fourteen Cambridge Center)		Office		Cambridge, MA	—					110		4,483		1,223			273		5,543		—		—		5,816		3,943		1983	-1
7300 Boston Boulevard, Building Thirteen		Office		Springfield, VA	—					608		4,773		230			661		4,950		—		—		5,611		4,229		2002	-1
8000 Corporate Court, Building Eleven		Office		Springfield, VA	—					136		3,071		1,611			775		4,043		—		—		4,818		2,494		1989	-1
7375 Boston Boulevard, Building Ten		Office		Springfield, VA	—					23		2,685		1,003			93		3,618		—		—		3,711		2,362		1988	-1
7374 Boston Boulevard, Building Four		Office		Springfield, VA	—					241		1,605		1,729			398		3,177		—		—		3,575		2,235		1984	-1
7451 Boston Boulevard, Building Two		Office		Springfield, VA	—					249		1,542		1,661			613		2,839		—		—		3,452		2,135		1982	-1
32 Hartwell Avenue		Office		Lexington, MA	—					168		1,943		809			314		2,606		—		—		2,920		1,685		1968-1979/1987	-1
164 Lexington Road		Office		Billerica, MA	—					592		1,370		319			643		1,638		—		—		2,281		745		1982	-1
17 Hartwell Avenue		Office		Lexington, MA	—					26		150		1,018			65		1,113		16		—		1,194		714		1968	-1
Residences on The Avenue,		Residential		Washington, DC	—					—		119,874		(28		)	—		119,846		—		—		119,846		11,039		2011	-1
2221 I St., NW
The Avant at Reston Town Center		Residential		Reston, VA	—					20,350		91,995		—			20,350		91,995		—		—		112,345		2,397		2014	-1
The Lofts at Atlantic Wharf		Residential		Boston, MA	—					3,529		54,891		1,543			3,529		56,434		—		—		59,963		5,093		2011	-1
Boston Marriott Cambridge (formerly Cambridge Center Marriott)		Hotel		Cambridge, MA	—					478		37,918		36,160			1,201		73,355		—		—		74,556		47,196		1986	-1
Cambridge Center East Garage		Garage		Cambridge, MA	—					—		35,035		1,561			103		36,493		—		—		36,596		7,647		1984	-1
Cambridge Center West Garage		Garage		Cambridge, MA	—					1,256		15,697		1,571			1,434		17,090		—		—		18,524		3,815		2006	-1
Cambridge Center North Garage		Garage		Cambridge, MA	—					1,163		11,633		2,770			1,579		13,987		—		—		15,566		8,531		1990	-1
Salesforce Tower		Development		San Francisco, CA	—					—		—		345,303			—		—		—		345,303		345,303		—		N/A	N/A
601 Massachusetts Avenue		Development		Washington, DC	—					—		—		189,173			12		—		—		189,161		189,173		—		N/A	N/A
535 Mission Street		Development		San Francisco, CA	—					—		—		171,096			10,789		39,307		—		121,000		171,096		225		N/A	N/A
10 CityPoint		Development		Waltham, MA	—					—		—		21,533			163		—		—		21,370		21,533		—		N/A	N/A
690 Folsom Street		Development		San Francisco, CA	—					—		—		13,237			1,777		6,546		—		4,914		13,237		11		N/A	N/A
Springfield Metro Center		Land		Springfield, VA	—					—		—		32,445			—		—		32,445		—		32,445		—		N/A	N/A
Reston Signature Site		Land		Reston, VA	—					—		—		30,256			—		—		30,256		—		30,256		—		N/A	N/A
Plaza at Almaden		Land		San Jose, CA	—					—		—		29,006			—		—		29,006		—		29,006		—		N/A	N/A
Tower Oaks Master Plan		Land		Rockville, MD	—					—		—		28,919			—		—		28,919		—		28,919		—		N/A	N/A
214 Third Avenue (formerly Prospect Hill)		Land		Waltham, MA	—					—		—		23,255			—		132		12,956		10,167		23,255		—		N/A	N/A
Washingtonian North		Land		Gaithersburg, MD	—					—		—		18,813			—		—		18,813		—		18,813		—		N/A	N/A
6601 & 6605 Springfield Center Drive		Land		Springfield, VA	—					—		—		13,866			—		—		13,866		—		13,866		—		N/A	N/A
103 Fourth Avenue		Land		Waltham, MA	—					—		—		11,920			—		—		11,920		—		11,920		—		N/A	N/A
Reston Gateway		Land		Reston, VA	—					—		—		9,933			—		—		9,933		—		9,933		—		N/A	N/A
Reston Eastgate		Land		Reston, VA	—					—		—		8,817			—		—		8,817		—		8,817		—		N/A	N/A
Crane Meadow		Land		Marlborough, MA	—					—		—		8,726			—		—		8,726		—		8,726		—		N/A	N/A
Broad Run Business Park		Land		Loudoun County, VA	—					—		—		6,311			—		—		6,311		—		6,311		—		N/A	N/A
20 CityPoint		Land		Waltham, MA	—					—		—		4,801			—		—		4,801		—		4,801		—		N/A	N/A
Cambridge Master Plan		Land		Cambridge, MA	—					—		—		3,527			—		—		3,527		—		3,527		—		N/A	N/A
North First Master Plan		Land		San Jose, CA	—					—		—		1,664			—		—		1,664		—		1,664		—		N/A	N/A
425 Fourth Street		Land		San Francisco, CA	—					—		—		1,261			—		—		1,261		—		1,261		—		N/A	N/A
30 Shattuck Road		Land		Andover, MA	—					—		—		1,213			—		—		1,213		—		1,213		—		N/A	N/A
					$	4,309,484		(2	)	$	4,661,817	$	11,331,324	$	3,215,276		$	4,785,772	$	13,418,220	$	268,114	$	736,311	$	19,208,417	$	3,529,978
Note: Total Real Estate does not include Furniture, Fixtures and Equipment totaling approximately $27,986. Accumulated Depreciation does not include approximately $17,681 of accumulated depreciation related to Furniture, Fixtures and Equipment.
The aggregate cost and accumulated depreciation for tax purposes was approximately $14.3 billion and $2.6 billion, respectively.
-1	Depreciation of the buildings and improvements are calculated over lives ranging from the life of the lease to 40 years.
-2	Includes the unamortized balance of the historical fair value adjustment totaling approximately $138.7 million.
A summary of activity for real estate and accumulated depreciation is as follows:
		2014			2013				2012
Real Estate:
Balance at the beginning of the year		$	18,953,601		$	14,869,887			$	13,363,113
Additions to/improvements of real estate		594,296			4,410,622				1,602,583
Assets sold/written-off		(339,480		)	(326,908		)		(95,809		)
Balance at the end of the year		$	19,208,417		$	18,953,601			$	14,869,887
Accumulated Depreciation:
Balance at the beginning of the year		$	3,145,701		$	2,919,479			$	2,626,324
Depreciation expense		456,176			419,908				367,625
Assets sold/written-off		(71,899		)	(193,686		)		(74,470		)
Balance at the end of the year		$	3,529,978		$	3,145,701			$	2,919,479
Note: Real Estate and Accumulated Depreciation amounts do not include Furniture, Fixtures and Equipment.

Summary_Of_Significant_Account1

Summary Of Significant Accounting Policies Summary Of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2014
Accounting Policies [Abstract]
Real Estate	Real Estate
	Upon acquisitions of real estate that constitutes a business, which includes the consolidation of previously unconsolidated joint ventures, the Company assesses the fair value of acquired tangible and intangible assets (including land, buildings, tenant improvements, “above-” and “below-market” leases, leasing and assumed financing origination costs, acquired in-place leases, other identified intangible assets and assumed liabilities) and allocates the purchase price to the acquired assets and assumed liabilities, including land and buildings as if vacant. The Company assesses and considers fair value based on estimated cash flow projections that utilize discount and/or capitalization rates that it deems appropriate, as well as available market information. Estimates of future cash flows are based on a number of factors including the historical operating results, known and anticipated trends, and market and economic conditions.
	The fair value of the tangible assets of an acquired property considers the value of the property as if it were vacant. The Company also considers an allocation of purchase price of other acquired intangibles, including acquired in-place leases that may have a customer relationship intangible value, including (but not limited to) the nature and extent of the existing relationship with the tenants, the tenants' credit quality and expectations of lease renewals. Based on its acquisitions to date, the Company's allocation to customer relationship intangible assets has been immaterial.
	The Company records acquired “above-” and “below-market” leases at their fair values (using a discount rate which reflects the risks associated with the leases acquired) equal to the difference between (1) the contractual amounts to be paid pursuant to each in-place lease and (2) management’s estimate of fair market lease rates for each corresponding in-place lease, measured over a period equal to the remaining term of the lease for above-market leases and the initial term plus the term of any below-market fixed rate renewal options for below- market leases. Acquired “above-” and “below-market” lease values have been reflected within Prepaid Expenses and Other Assets and Other Liabilities, respectively, in the Company's Consolidated Balance Sheets. Other intangible assets acquired include amounts for in-place lease values that are based on the Company's evaluation of the specific characteristics of each tenant's lease. Factors to be considered include estimates of carrying costs during hypothetical expected lease-up periods considering current market conditions, and costs to execute similar leases. In estimating carrying costs, the Company includes real estate taxes, insurance and other operating expenses and estimates of lost rentals at market rates during the expected lease-up periods, depending on local market conditions. In estimating costs to execute similar leases, the Company considers leasing commissions, legal and other related expenses.
	Management reviews its long-lived assets for impairment following the end of each quarter and when there is an event or change in circumstances that indicates an impairment in value. An impairment loss is recognized if the carrying amount of its assets is not recoverable and exceeds its fair value. If such criteria are present, an impairment loss is recognized based on the excess of the carrying amount of the asset over its fair value. The evaluation of anticipated cash flows is highly subjective and is based in part on assumptions regarding future occupancy, rental rates and capital requirements that could differ materially from actual results in future periods. Since cash flows on properties considered to be “long-lived assets to be held and used” are considered on an undiscounted basis to determine whether an asset has been impaired, the Company's established strategy of holding properties over the long term directly decreases the likelihood of recording an impairment loss. If the Company's strategy changes or market conditions otherwise dictate an earlier sale date, an impairment loss may be recognized and such loss could be material. If the Company determines that an impairment has occurred, the affected assets must be reduced to their fair value, less cost to sell.
	Guidance in Accounting Standards Codification (“ASC”) 360 “Property Plant and Equipment” (“ASC 360”) requires that qualifying assets and liabilities and the results of operations that have been sold, or otherwise qualify as “held for sale,” be presented as discontinued operations in all periods presented if the property operations are expected to be eliminated and the Company will not have significant continuing involvement following the sale. The components of the property’s net income that are reflected as discontinued operations include the net gain (or loss) upon the disposition of the property held for sale, operating results, depreciation and interest expense (if the property is subject to a secured loan). The Company generally considers assets to be “held for sale” when the transaction has been approved by the Board of Directors, or a committee thereof, and there are no known significant contingencies relating to the sale, such that a sale of the property within one year is considered probable. Following the classification of a property as “held for sale,” no further depreciation is recorded on the assets, and the asset is written down to the lower of carrying value or fair market value, less cost to sell. On April 10, 2014, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) 2014-08, "Reporting Discontinued Operations and Disclosures of Disposals of Components of an Entity" (“ASU 2014-08”). ASU 2014-08 clarifies that discontinued operations presentation applies only to disposals representing a strategic shift that has (or will have) a major effect on an entity’s operations and financial results (e.g., a disposal of a major geographical area, a major line of business, a major equity method investment or other major parts of an entity). ASU 2014-08 is effective prospectively for reporting periods beginning after December 15, 2014. Early adoption is permitted, and the Company early adopted ASU 2014-08 during the first quarter of 2014. The Company’s adoption of ASU 2014-08 resulted in the operating results and gains on sales of real estate from operating properties sold during the year ended December 31, 2014 not being reflected within Discontinued Operations in the Company's Consolidated Statements of Operations (See Note 3).
	Real estate is stated at depreciated cost. A variety of costs are incurred in the acquisition, development and leasing of properties. The cost of buildings and improvements includes the purchase price of property, legal fees and other acquisition costs. The Company expenses costs that it incurs to effect a business combination such as legal, due diligence and other closing related costs. Costs directly related to the development of properties are capitalized. Capitalized development costs include interest, internal wages, property taxes, insurance, and other project costs incurred during the period of development. After the determination is made to capitalize a cost, it is allocated to the specific component of a project that is benefited. Determination of when a development project commences and capitalization begins, and when a development project is substantially complete and held available for occupancy and capitalization must cease, involves a degree of judgment. The Company’s capitalization policy on development properties is guided by guidance in ASC 835-20 “Capitalization of Interest” and ASC 970 “Real Estate-General.” The costs of land and buildings under development include specifically identifiable costs.
	The capitalized costs include pre-construction costs necessary to the development of the property, development costs, construction costs, interest costs, real estate taxes, salaries and related costs and other costs incurred during the period of development. The Company begins the capitalization of costs during the pre-construction period which it defines as activities that are necessary to the development of the property. The Company considers a construction project as substantially completed and held available for occupancy upon the completion of tenant improvements, but no later than one year from cessation of major construction activity. The Company ceases capitalization on the portion (1) substantially completed, (2) occupied or held available for occupancy, and capitalizes only those costs associated with the portion under construction or (3) if activities necessary for the development of the property have been suspended. Interest costs capitalized for the years ended December 31, 2014, 2013 and 2012 were $52.5 million, $68.2 million and $44.3 million, respectively. Salaries and related costs capitalized for the years ended December 31, 2014, 2013 and 2012 were $8.5 million, $7.7 million and $7.1 million, respectively.
	Expenditures for repairs and maintenance are charged to operations as incurred. Significant betterments are capitalized. When assets are sold or retired, their costs and related accumulated depreciation are removed from the accounts with the resulting gains or losses reflected in net income or loss for the period.
	The Company computes depreciation and amortization on properties using the straight-line method based on estimated useful asset lives. In accordance with ASC 805, the Company allocates the acquisition cost of real estate to its components and depreciates or amortizes these assets (or liabilities) over their useful lives. The amortization of acquired “above-” and “below-market” leases and acquired in-place leases is recorded as an adjustment to revenue and depreciation and amortization, respectively, in the Consolidated Statements of Operations.
	Depreciation is computed on a straight-line basis over the estimated useful lives of the assets as follows:
	Land improvements		25 to 40 years
	Buildings and improvements		10 to 40 years
	Tenant improvements		Shorter of useful life or terms of related lease
	Furniture, fixtures, and equipment		3 to 7 years
Cash and Cash Equivalents	Cash and Cash Equivalents
	Cash and cash equivalents consist of cash on hand and investments with maturities of three months or less from the date of purchase. The majority of the Company’s cash and cash equivalents are held at major commercial banks which may at times exceed the Federal Deposit Insurance Corporation limit of $250,000. The Company has not experienced any losses to date on its invested cash.
Cash Held in Escrows	Cash Held in Escrows
	Escrows include amounts established pursuant to various agreements for security deposits, property taxes, insurance and other costs. Escrows also include cash held by qualified intermediaries for possible investments in a like-kind exchanges in accordance with Section 1031 of the Internal Revenue Code in connection with sales of the Company’s properties.
Investments in Securities	Investments in Securities
	The Company accounts for investments in trading securities at fair value, with gains or losses resulting from changes in fair value recognized currently in earnings. The designation of trading securities is generally determined at acquisition. The Company maintains a deferred compensation plan that is designed to allow officers of the Company to defer a portion of their current income on a pre-tax basis and receive a tax-deferred return on these deferrals. The Company’s obligation under the plan is that of an unsecured promise to pay the deferred compensation to the plan participants in the future. At December 31, 2014 and 2013, the Company had maintained approximately $19.5 million and $16.6 million, respectively, in a separate account, which is not restricted as to its use. The Company recognized gains of approximately $1.0 million, $2.9 million and $1.4 million on its investments in the account associated with the Company’s deferred compensation plan during the years ended December 31, 2014, 2013 and 2012, respectively.
Tenant and Other Receivables	Tenant and Other Receivables
	Tenant and other accounts receivable, other than accrued rents receivable, are expected to be collected within one year.
Deferred Charges	Deferred Charges
	Deferred charges include leasing costs and financing fees. Leasing costs include an allocation for acquired intangible in-place lease values and direct and incremental fees and costs incurred in the successful negotiation of leases, including brokerage, legal, internal leasing employee salaries and other costs which have been deferred and are being amortized on a straight-line basis over the terms of the respective leases. Internal leasing salaries and related costs capitalized for the years ended December 31, 2014, 2013 and 2012 were $6.0 million, $5.1 million and $5.6 million, respectively. External fees and costs incurred to obtain long-term financing have been deferred and are being amortized over the terms of the respective loans and are included within interest expense. Unamortized financing and leasing costs are charged to expense upon the early repayment or significant modification of the financing or upon the early termination of the lease, respectively. Fully amortized deferred charges are removed from the books upon the expiration of the lease or maturity of the debt.
Investments in Unconsolidated Joint Ventures	Investments in Unconsolidated Joint Ventures
	The Company consolidates variable interest entities (“VIEs”) in which it is considered to be the primary beneficiary. VIEs are entities in which the equity investors do not have sufficient equity at risk to finance their endeavors without additional financial support or that the holders of the equity investment at risk do not have a controlling financial interest. The primary beneficiary is defined by the entity having both of the following characteristics: (1) the power to direct the activities that, when taken together, most significantly impact the variable interest entity's performance, and (2) the obligation to absorb losses and right to receive the returns from the variable interest entity that would be significant to the variable interest entity. For ventures that are not VIEs the Company consolidates entities for which it has significant decision making control over the ventures' operations. The Company's judgment with respect to its level of influence or control of an entity involves the consideration of various factors including the form of the Company's ownership interest, its representation in the entity's governance, the size of its investment (including loans), estimates of future cash flows, its ability to participate in policy making decisions and the rights of the other investors to participate in the decision making process and to replace the Company as manager and/or liquidate the venture, if applicable. The Company's assessment of its influence or control over an entity affects the presentation of these investments in the Company's consolidated financial statements. In addition to evaluating control rights, the Company consolidates entities in which the outside partner has no substantive kick-out rights to remove the Company as the managing member.
	Accounts of the consolidated entity are included in the accounts of the Company and the non-controlling interest is reflected on the Consolidated Balance Sheets as a component of equity or in temporary equity between liabilities and equity. Investments in unconsolidated joint ventures are recorded initially at cost, and subsequently adjusted for equity in earnings and cash contributions and distributions. Any difference between the carrying amount of these investments on the balance sheet and the underlying equity in net assets is amortized as an adjustment to equity in earnings of unconsolidated joint ventures over the life of the related asset. Under the equity method of accounting, the net equity investment of the Company is reflected within the Consolidated Balance Sheets, and the Company's share of net income or loss from the joint ventures is included within the Consolidated Statements of Operations. The joint venture agreements may designate different percentage allocations among investors for profits and losses; however, the Company's recognition of joint venture income or loss generally follows the joint venture's distribution priorities, which may change upon the achievement of certain investment return thresholds. The Company may account for cash distributions in excess of its investment in an unconsolidated joint venture as income when the Company is not the general partner in a limited partnership and when the Company has neither the requirement nor the intent to provide financial support to the joint venture. The Company's investments in unconsolidated joint ventures are reviewed for impairment periodically and the Company records impairment charges when events or circumstances change indicating that a decline in the fair values below the carrying values has occurred and such decline is other-than-temporary. The ultimate realization of the investment in unconsolidated joint ventures is dependent on a number of factors, including the performance of each investment and market conditions. The Company will record an impairment charge if it determines that a decline in the value below the carrying value of an investment in an unconsolidated joint venture is other-than-temporary.
	To the extent that the Company contributes assets to a joint venture, the Company’s investment in the joint venture is recorded at the Company’s cost basis in the assets that were contributed to the joint venture. To the extent that the Company’s cost basis is different than the basis reflected at the joint venture level, the basis difference is amortized over the life of the related asset and included in the Company’s share of equity in net income of the joint venture. In accordance with the provisions of ASC 970-323 “Investments—Equity Method and Joint Ventures” (“ASC 970-323”), the Company will recognize gains on the contribution of real estate to joint ventures, relating solely to the outside partner’s interest, to the extent the economic substance of the transaction is a sale.
	The combined summarized financial information of the unconsolidated joint ventures is disclosed in Note 5 to the Consolidated Financial Statements.
Equity Offering Costs	Equity Offering Costs
	Underwriting commissions and offering costs have been reflected as a reduction of additional paid-in capital
Treasury Stock	Treasury Stock
	The Company’s share repurchases are reflected as treasury stock utilizing the cost method of accounting and are presented as a reduction to consolidated stockholders’ equity.
Dividends	Dividends
	Earnings and profits, which determine the taxability of dividends to stockholders, will differ from income reported for financial reporting purposes due to the differences for federal income tax purposes in the treatment of gains/losses on the sale of real property, revenue and expense recognition, compensation expense, and in the estimated useful lives and basis used to compute depreciation.
	The tax treatment of common dividends per share for federal income tax purposes is as follows:
			For the year ended December 31,
			2014							2013								2012
			Per Share				%			Per Share				%				Per Share			%
	Ordinary income		$	—			—	%		$	2.31			48.71	%	0.9093		$	2.29	0.9093	96.45	%
	Capital gain income		6.82				100	%		2.44				51.29	%	—		0.08		—	3.55	%
	Total		$	6.82		-1	100	%		$	4.75		-2	100	%			$	2.37	1	100	%
	_____________
	(1) The fourth quarter 2014 dividend of $5.15 per common share consists of a $4.50 per common share special dividend and a $0.65 per common share regular quarterly dividend, approximately $4.41 per common share was allocable to 2014 and approximately $0.74 per common share is allocable to 2015.
	(2) The fourth quarter 2013 dividend of $2.90 per common share consists of a $2.25 per common share special dividend and a $0.65 per common share regular quarterly dividend, approximately $2.44 per common share was allocable to 2013 and approximately $0.46 per common share is allocable to 2014.
Revenue Recognition	Revenue Recognition
	In general, the Company commences rental revenue recognition when the tenant takes possession of the leased space and the leased space is substantially ready for its intended use. Contractual rental revenue is reported on a straight-line basis over the terms of the respective leases. The impact of the straight-line rent adjustment increased revenue by approximately $63.1 million, $65.8 million and $77.6 million for the years ended December 31, 2014, 2013 and 2012, respectively, as the revenue recorded exceeded amounts billed. Accrued rental income, as reported on the Consolidated Balance Sheets, represents cumulative rental income earned in excess of rent payments received pursuant to the terms of the individual lease agreements. The Company maintains an allowance against accrued rental income for future potential tenant credit losses. The credit assessment is based on the estimated accrued rental income that is recoverable over the term of the lease. The Company also maintains an allowance for doubtful accounts for estimated losses resulting from the inability of tenants to make required rent payments. The computation of this allowance is based on the tenants’ payment history and current credit status, as well as certain industry or geographic specific credit considerations. If the Company’s estimates of collectability differ from the cash received, then the timing and amount of the Company’s reported revenue could be impacted. The credit risk is mitigated by the high quality of the Company’s existing tenant base, reviews of prospective tenants’ risk profiles prior to lease execution and consistent monitoring of the Company’s portfolio to identify potential problem tenants.
	In accordance with ASC 805, the Company recognizes rental revenue of acquired in-place “above-” and “below-market” leases at their fair values over the original term of the respective leases. The impact of the acquired in-place “above-” and “below-market” leases increased revenue by approximately $48.3 million, $28.0 million and $14.6 million for the years ended December 31, 2014, 2013 and 2012, respectively. The following table summarizes the scheduled amortization of the Company's acquired “above-” and “below-market” lease intangibles for each of the five succeeding years (in thousands). Accrued rental income as reported on the Consolidated Balance Sheets represents rental income recognized in excess of rent payments actually received pursuant to the terms of the individual lease agreements.
			Acquired Above-Market Lease Intangibles				Acquired Below-Market Lease Intangibles
	2015		$	22,671			$	57,019
	2016		20,491				51,460
	2017		12,277				35,896
	2018		8,637				33,215
	2019		7,106				27,615
	Recoveries from tenants, consisting of amounts due from tenants for common area maintenance, real estate taxes and other recoverable costs, are recognized as revenue in the period during which the expenses are incurred. Tenant reimbursements are recognized and presented in accordance with guidance in ASC 605-45 “Principal Agent Considerations” (“ASC 605-45”). ASC 605-45 requires that these reimbursements be recorded on a gross basis, as the Company is generally the primary obligor with respect to purchasing goods and services from third-party suppliers, has discretion in selecting the supplier and has credit risk. The Company also receives reimbursement of payroll and payroll related costs from third parties which the Company reflects on a net basis.
	The Company's parking revenues are derived from leases, monthly parking and transient parking. The Company recognizes parking revenue as earned.
	The Company’s hotel revenues are derived from room rentals and other sources such as charges to guests for telephone service, movie and vending commissions, meeting and banquet room revenue and laundry services. Hotel revenues are recognized as earned.
	The Company receives management and development fees from third parties. Property management fees are recorded and earned based on a percentage of collected rents at the properties under management, and not on a straight-line basis, because such fees are contingent upon the collection of rents. The Company records development fees as earned depending on the risk associated with each project. Profit on development fees earned from joint venture projects is recognized as revenue to the extent of the third party partners’ ownership interest.
	Gains on sales of real estate are recognized pursuant to the provisions included in ASC 360-20 “Real Estate Sales” (“ASC 360-20”). The specific timing of the sale is measured against various criteria in ASC 360-20 related to the terms of the transaction and any continuing involvement in the form of management or financial assistance associated with the properties. If the sales criteria for the full accrual method are not met, the Company defers some or all of the gain recognition and accounts for the continued operations of the property by applying the finance, leasing, profit sharing, deposit, installment or cost recovery methods, as appropriate, until the sales criteria are met.
Depreciation and Amortization	Depreciation and Amortization
	The Company computes depreciation and amortization on its properties using the straight-line method based on estimated useful asset lives. The Company allocates the acquisition costs of real estate to its components and depreciate or amortize these assets over their useful lives. The amortization of acquired “above-” and “below-market” leases and acquired in-place leases is recorded as an adjustment to revenue and depreciation and amortization, respectively, in the Consolidated Statements of Operations.
Ground Leases	Ground Leases
	The Company has non-cancelable ground lease obligations with various initial term expiration dates through 2068 (See Note 20). The Company recognizes ground rent expense on a straight-line basis over the terms of the respective ground lease agreements. The future contractual minimum lease payments to be made by the Company as of December 31, 2014, under non-cancelable ground leases which expire on various dates through 2068, are as follows:
	Years Ending December 31,	(in thousands)
	2015	$	13,507
	2016	13,732
	2017	13,963
	2018	14,198
	2019	14,461
	Thereafter	884,726
Earnings Per Share	Earnings Per Share
	Basic earnings per share (“EPS”) is computed by dividing net income available to common shareholders, as adjusted for undistributed earnings (if any) of certain securities issued by the Operating Partnership, by the weighted average number of shares of Common Stock outstanding during the year. Diluted EPS reflects the potential dilution that could occur from shares issuable in connection with awards under stock-based compensation plans, including upon the exercise of stock options, and securities of the Operating Partnership that are exchangeable for Common Stock.
Fair Value of Financial Instruments	Fair Value of Financial Instruments
	The carrying values of cash and cash equivalents, marketable securities, escrows, receivables, accounts payable, accrued expenses and other assets and liabilities are reasonable estimates of their fair values because of the short maturities of these instruments.
	The Company follows the authoritative guidance for fair value measurements when valuing its financial instruments for disclosure purposes. The Company determines the fair value of its unsecured senior notes and unsecured exchangeable senior notes using market prices. The inputs used in determining the fair value of the Company’s unsecured senior notes and unsecured exchangeable senior notes is categorized at a level 1 basis (as defined in the accounting standards for Fair Value Measurements and Disclosures) due to the fact that the Company uses quoted market rates to value these instruments. However, the inputs used in determining the fair value could be categorized at a level 2 basis if trading volumes are low. The Company determines the fair value of its mortgage notes payable using discounted cash flow analyses by discounting the spread between the future contractual interest payments and hypothetical future interest payments on mortgage debt based on current market rates for similar securities. In determining the current market rates, the Company adds its estimates of market spreads to the quoted yields on federal government treasury securities with similar maturity dates to its debt. The inputs used in determining the fair value of the Company’s mortgage notes payable and mezzanine notes payable are categorized at a level 3 basis (as defined in the accounting standards for Fair Value Measurements and Disclosures) due to the fact that the Company considers the rates used in the valuation techniques to be unobservable inputs.
	Because the Company’s valuations of its financial instruments are based on these types of estimates, the actual fair values of its financial instruments may differ materially if the Company’s estimates do not prove to be accurate. The following table presents the aggregate carrying value of the Company’s indebtedness and the Company’s corresponding estimate of fair value as of December 31, 2014 and December 31, 2013 (in thousands):
		December 31, 2014								December 31, 2013
		Carrying				Estimated				Carrying						Estimated
		Amount				Fair Value				Amount						Fair Value
	Mortgage notes payable	$	4,309,484			$	4,449,541			$	4,449,734					$	4,545,283
	Mezzanine notes payable	309,796				306,156				311,040						311,064
	Unsecured senior notes	5,287,704				5,645,819				5,835,854						6,050,517
	Unsecured exchangeable senior notes	—				—				744,880				-1		750,266
	Total	$	9,906,984			$	10,401,516			$	11,341,508					$	11,657,130
	_______________
	-1	Includes the net adjustment for the equity component allocation totaling approximately $2.4 million at December 31, 2013.
Derivative Instruments and Hedging Activities	Derivative Instruments and Hedging Activities
	Derivative instruments and hedging activities require management to make judgments on the nature of its derivatives and their effectiveness as hedges. These judgments determine if the changes in fair value of the derivative instruments are reported in the Consolidated Statements of Operations as a component of net income or as a component of comprehensive income and as a component of equity on the Consolidated Balance Sheets. While management believes its judgments are reasonable, a change in a derivative’s effectiveness as a hedge could materially affect expenses, net income and equity. The Company accounts for the effective portion of changes in the fair value of a derivative in other comprehensive income (loss) and subsequently reclassifies the effective portion to earnings over the term that the hedged transaction affects earnings. The Company accounts for the ineffective portion of changes in the fair value of a derivative directly in earnings.
Income Taxes	Income Taxes
	The Company has elected to be treated as a REIT under Sections 856 through 860 of the Internal Revenue Code of 1986, as amended (the “Code”), commencing with its taxable year ended December 31, 1997. As a result, the Company generally will not be subject to federal corporate income tax on its taxable income that is distributed to its stockholders. A REIT is subject to a number of organizational and operational requirements, including a requirement that it currently distribute at least 90% of its annual taxable income (with certain adjustments). The Company’s policy is to distribute at least 100% of its taxable income. Accordingly, the only provision for federal income taxes in the accompanying consolidated financial statements relates to the Company’s consolidated taxable REIT subsidiaries. The Company’s taxable REIT subsidiaries did not have significant tax provisions or deferred income tax items. The Company has no uncertain tax positions recognized as of December 31, 2014 and 2013.
	The Company owns a hotel property which is managed through a taxable REIT subsidiary. The hotel taxable REIT subsidiary, a wholly owned subsidiary of the Operating Partnership, is the lessee pursuant to the lease for the hotel property. As lessor, the Operating Partnership is entitled to a percentage of gross receipts from the hotel property. Marriott International, Inc. continues to manage the hotel property under the Marriott name and under terms of the existing management agreement. The hotel taxable REIT subsidiary is subject to tax at the federal and state level and, accordingly, the Company has recorded a tax provision in the Company’s Consolidated Statements of Operations for the years ended December 31, 2014, 2013 and 2012.
	The net difference between the tax basis and the reported amounts of the Company’s assets and liabilities is approximately $2.2 billion and $1.7 billion as of December 31, 2014 and 2013, respectively, which is primarily related to the difference in basis of contributed property and accrued rental income.
	Certain entities included in the Company’s consolidated financial statements are subject to certain state and local taxes. These taxes are recorded as operating expenses in the accompanying consolidated financial statements.
	The following table reconciles GAAP net income attributable to Boston Properties, Inc. to taxable income:
			For the year ended December 31,
			2014				2013				2012
			(in thousands)
	Net income attributable to Boston Properties, Inc.		$	443,611			$	749,811			$	289,650
	Straight-line rent adjustments		(91,733		)		(74,445		)		(80,281		)
	Book/Tax differences from depreciation and amortization		239,681				170,370				105,599
	Book/Tax differences from interest expense		(43,148		)		(7,912		)		27,660
	Book/Tax differences on gains/losses from capital transactions		943,778				(124,413		)		(22,408		)
	Book/Tax differences from stock-based compensation		32,483				42,146				19,660
	Tangible Property Regulations (1)		(442,650		)		—				—
	Other book/tax differences, net		(7,945		)		(4,885		)		7,801
	Taxable income		$	1,074,077			$	750,672			$	347,681
	__________
	(1) In September 2013, the Internal Revenue Service released final Regulations governing when taxpayers like the Company must capitalize and depreciate costs for acquiring, maintaining, repairing and replacing tangible property and when taxpayers can deduct such costs.  These final Regulations are effective for tax years beginning on or after January 1, 2014.  These Regulations permitted the Company to deduct certain types of expenditures that were previously required to be capitalized.  The Regulations also allowed the Company to make a one-time election to immediately deduct certain amounts that were capitalized in previous years that are not required to be capitalized under the new Regulations. The one-time deduction included above totaled approximately $385.6 million.
Share-Based Employee Compensation Plans	Stock-Based Employee Compensation Plans
	At December 31, 2014, the Company has a stock-based employee compensation plan. Effective January 1, 2005, the Company adopted early ASC 718 “Compensation – Stock Compensation” (“ASC 718”), which revised the fair value based method of accounting for share-based payment liabilities, forfeitures and modifications of stock-based awards and clarified previous guidance in several areas, including measuring fair value, classifying an award as equity or as a liability and attributing compensation cost to reporting periods.
Use of Estimatesin the Preperation of Financial Statements	Use of Estimates in the Preparation of Financial Statements
	The preparation of financial statements in conformity with accounting principles generally accepted in the United States of America requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of revenue and expenses during the reporting period. These estimates include such items as depreciation and allowances for doubtful accounts. Actual results could differ from those estimates.
Recent Accounting Pronouncements	Recent Accounting Pronouncements
	On April 10, 2014, the FASB issued ASU 2014-08, "Reporting Discontinued Operations and Disclosures of Disposals of Components of an Entity" (“ASU 2014-08”). ASU 2014-08 clarifies that discontinued operations presentation applies only to disposals representing a strategic shift that has (or will have) a major effect on an entity’s operations and financial results (e.g., a disposal of a major geographical area, a major line of business, a major equity method investment or other major parts of an entity). ASU 2014-08 is effective prospectively for reporting periods beginning after December 15, 2014. Early adoption is permitted, and the Company early adopted ASU 2014-08 during the first quarter of 2014. The Company’s adoption of ASU 2014-08 resulted in the operating results and gains on sales of real estate from operating properties sold during the year ended December 31, 2014 not being reflected within Discontinued Operations in the Company's Consolidated Statements of Operations (See Note 3).
	In May 2014, the FASB issued ASU 2014-09, "Revenue from Contracts with Customers (Topic 606)" ("ASU 2014-09"). The objective of ASU 2014-09 is to establish a single comprehensive model for entities to use in accounting for revenue arising from contracts with customers and will supersede most of the existing revenue recognition guidance, including industry-specific guidance. The core principle is that an entity should recognize revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. In applying ASU 2014-09, companies will perform a five-step analysis of transactions to determine when and how revenue is recognized. ASU 2014-09 applies to all contracts with customers except those that are within the scope of other topics in the FASB's Accounting Standards Codification ("ASC"). ASU 2014-09 is effective for annual reporting periods (including interim periods within that reporting period) beginning after December 15, 2016 and shall be applied using either a full retrospective or modified retrospective approach. Early adoption is not permitted. The Company is currently assessing the potential impact that the adoption of ASU 2014-09 will have on its consolidated financial statements.
	In June 2014, the FASB issued ASU 2014-12, “Accounting for Share-Based Payments When the Terms of an Award Provide That a Performance Target Could Be Achieved after the Requisite Service Period” (“ASU 2014-12”). The amendments in ASU 2014-12 require that a performance target that affects vesting and that could be achieved after the requisite service period be treated as a performance condition. A reporting entity should apply existing guidance in ASC Topic No. 718, “Compensation - Stock Compensation” (“ASC 718”), as it relates to awards with performance conditions that affect vesting to account for such awards. The amendments in ASU 2014-12 are effective for annual periods and interim periods within those annual periods beginning after December 15, 2015. Early adoption is permitted. Entities may apply the amendments in ASU 2014-12 either: (a) prospectively to all awards granted or modified after the effective date; or (b) retrospectively to all awards with performance targets that are outstanding as of the beginning of the earliest annual period presented in the financial statements and to all new or modified awards thereafter. The Company does not expect the adoption of ASU 2014-12 to have a material impact on its consolidated financial statements.
	In August 2014, the FASB issued ASU 2014-15, "Presentation of Financial Statements - Going Concern: Disclosure of Uncertainties about an Entity’s Ability to Continue as a Going Concern" (“ASU 2014-15”). ASU 2014-15 requires an entity to evaluate whether there are conditions or events that raise substantial doubt about the entity’s ability to continue as a going concern within one year after the date that the financial statements are issued (or within one year after the financial statements are available to be issued when applicable) and to provide related footnote disclosures in certain circumstances. ASU 2014-15 is effective for the annual period ending after December 15, 2016, and for annual and interim periods thereafter with early adoption permitted. The Company does not expect the adoption of ASU 2014-15 to have a material impact on its consolidated financial statements.
	In November 2014, the FASB issued ASU 2014-16, "Determining Whether the Host Contract in a Hybrid Financial Instrument Issued in the Form of a Share Is More Akin to Debt or to Equity" (“ASU 2014-16”). ASU 2014-16 clarifies how current GAAP should be interpreted in evaluating the economic characteristics and risks of a host contract in a hybrid financial instrument that is issued in the form of a share. Specifically, the amendments clarify that an entity should consider all relevant terms and features—including the embedded derivative feature being evaluated for bifurcation—in evaluating the nature of the host contract. Furthermore, the amendments clarify that no single term or feature would necessarily determine the economic characteristics and risks of the host contract. Rather, the nature of the host contract depends upon the economic characteristics and risks of the entire hybrid financial instrument. ASU 2014-16 is effective for fiscal years and interim periods beginning after December 15, 2015. Early adoption is permitted. The Company does not expect the adoption of ASU 2014-16 to have a material impact on its consolidated financial statements.
	In February 2015, the FASB issued ASU 2015-02, “Consolidation (Topic 810): Amendments to the Consolidation Analysis” (“ASU 2015-02”). ASU 2015-02 affects reporting entities that are required to evaluate whether they should consolidate certain legal entities. ASU 2015-02 modifies the evaluation of whether limited partnerships and similar legal entities are VIEs or voting interest entities, eliminates the presumption that a general partner should consolidate a limited partnership and affects the consolidation analysis of reporting entities that are involved with VIEs, particularly those that have fee arrangements and related party relationships. ASU 2015-02 is effective for fiscal years, and for interim periods within those fiscal years, beginning after December 15, 2015. Early adoption is permitted. A reporting entity may apply the amendments in ASU 2015-02 using: (a) a modified retrospective approach by recording a cumulative-effect adjustment to equity as of the beginning of the fiscal year of adoption; or (b) by applying the amendments retrospectively. The Company is currently assessing the potential impact that the adoption of ASU 2015-02 will have on its consolidated financial statements.portion

Summary_Of_Significant_Account2

Summary Of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2014
Accounting Policies [Abstract]
Fair Value, by Balance Sheet Grouping [Table Text Block]
		December 31, 2014								December 31, 2013
		Carrying				Estimated				Carrying						Estimated
		Amount				Fair Value				Amount						Fair Value
	Mortgage notes payable	$	4,309,484			$	4,449,541			$	4,449,734					$	4,545,283
	Mezzanine notes payable	309,796				306,156				311,040						311,064
	Unsecured senior notes	5,287,704				5,645,819				5,835,854						6,050,517
	Unsecured exchangeable senior notes	—				—				744,880				-1		750,266
	Total	$	9,906,984			$	10,401,516			$	11,341,508					$	11,657,130
	_______________
	-1	Includes the net adjustment for the equity component allocation totaling approximately $2.4 million at December 31, 2013.
Gaap Reconciliation Of Net Income To Taxable Income, Table [Table Text Block]	The following table reconciles GAAP net income attributable to Boston Properties, Inc. to taxable income:
			For the year ended December 31,
			2014				2013				2012
			(in thousands)
	Net income attributable to Boston Properties, Inc.		$	443,611			$	749,811			$	289,650
	Straight-line rent adjustments		(91,733		)		(74,445		)		(80,281		)
	Book/Tax differences from depreciation and amortization		239,681				170,370				105,599
	Book/Tax differences from interest expense		(43,148		)		(7,912		)		27,660
	Book/Tax differences on gains/losses from capital transactions		943,778				(124,413		)		(22,408		)
	Book/Tax differences from stock-based compensation		32,483				42,146				19,660
	Tangible Property Regulations (1)		(442,650		)		—				—
	Other book/tax differences, net		(7,945		)		(4,885		)		7,801
	Taxable income		$	1,074,077			$	750,672			$	347,681
	__________
	(1) In September 2013, the Internal Revenue Service released final Regulations governing when taxpayers like the Company must capitalize and depreciate costs for acquiring, maintaining, repairing and replacing tangible property and when taxpayers can deduct such costs.  These final Regulations are effective for tax years beginning on or after January 1, 2014.  These Regulations permitted the Company to deduct certain types of expenditures that were previously required to be capitalized.  The Regulations also allowed the Company to make a one-time election to immediately deduct certain amounts that were capitalized in previous years that are not required to be capitalized under the new Regulations. The one-time deduction included above totaled approximately $385.6 million.
Schedule of Finite-Lived Intangible Assets, Future Amortization Expense [Table Text Block]	The following table summarizes the scheduled amortization of the Company's acquired “above-” and “below-market” lease intangibles for each of the five succeeding years (in thousands). Accrued rental income as reported on the Consolidated Balance Sheets represents rental income recognized in excess of rent payments actually received pursuant to the terms of the individual lease agreements.
			Acquired Above-Market Lease Intangibles				Acquired Below-Market Lease Intangibles
	2015		$	22,671			$	57,019
	2016		20,491				51,460
	2017		12,277				35,896
	2018		8,637				33,215
	2019		7,106				27,615
	The following table summarizes the scheduled amortization of the Company's acquired in-place lease intangibles for each of the five succeeding years (in thousands).
			Acquired In-Place Lease Intangibles
	2015		$	66,390
	2016		55,327
	2017		38,812
	2018		33,964
	2019		27,439
Schedule Of Future Contractual Minimum Lease Payments Under Non-Cancelable Ground Leases [Table Text Block]
	Years Ending December 31,	(in thousands)
	2015	$	13,507
	2016	13,732
	2017	13,963
	2018	14,198
	2019	14,461
	Thereafter	884,726
Tax Treatment Of Common Dividends Per Share For Federal Tax Purposes, Table [Table Text Block]	The tax treatment of common dividends per share for federal income tax purposes is as follows:
			For the year ended December 31,
			2014							2013								2012
			Per Share				%			Per Share				%				Per Share			%
	Ordinary income		$	—			—	%		$	2.31			48.71	%	0.9093		$	2.29	0.9093	96.45	%
	Capital gain income		6.82				100	%		2.44				51.29	%	—		0.08		—	3.55	%
	Total		$	6.82		-1	100	%		$	4.75		-2	100	%			$	2.37	1	100	%
	_____________
	(1) The fourth quarter 2014 dividend of $5.15 per common share consists of a $4.50 per common share special dividend and a $0.65 per common share regular quarterly dividend, approximately $4.41 per common share was allocable to 2014 and approximately $0.74 per common share is allocable to 2015.
	(2) The fourth quarter 2013 dividend of $2.90 per common share consists of a $2.25 per common share special dividend and a $0.65 per common share regular quarterly dividend, approximately $2.44 per common share was allocable to 2013 and approximately $0.46 per common share is allocable to 2014.
Property, Plant and Equipment [Table Text Block]	Depreciation is computed on a straight-line basis over the estimated useful lives of the assets as follows:
	Land improvements		25 to 40 years
	Buildings and improvements		10 to 40 years
	Tenant improvements		Shorter of useful life or terms of related lease
	Furniture, fixtures, and equipment		3 to 7 years

Real_Estate_Tables

Real Estate (Tables)	12 Months Ended
	Dec. 31, 2014									Dec. 31, 2013
Real Estate Properties [Line Items]
Schedule of Property Subject to or Available for Operating Lease [Table Text Block]	Real estate consisted of the following at December 31 (in thousands):
			2014				2013
	Land		$	4,785,772			$	4,450,532
	Land held for future development		268,114				297,376
	Buildings and improvements		11,666,105				11,065,113
	Tenant improvements		1,752,115				1,617,401
	Furniture, fixtures and equipment		27,986				25,164
	Construction in progress		736,311				1,523,179
	Total		19,236,403				18,978,765
	Less: Accumulated depreciation		(3,547,659		)		(3,161,571		)
			$	15,688,744			$	15,817,194
Pro Forma In Connection With Acquisition	The accompanying unaudited pro forma information for the years ended December 31, 2013 and 2012 is presented as if the operating property acquisitions of (1) Mountain View Research Park and Mountain View Technology Park on April 10, 2013 and the approximately $26.5 million gain on consolidation and (2) 767 Fifth Avenue (the General Motors Building) on May 31, 2013 and the approximately $359.5 million gain on consolidation, had occurred on January 1, 2012. This unaudited pro forma information is based upon the historical consolidated financial statements of the Company and should be read in conjunction with the consolidated financial statements and notes thereto. This pro forma information does not purport to represent what the actual results of operations of the Company would have been had the above occurred, nor do they purport to predict the results of operations of future periods. Additional information for these transactions are provided below.
	Pro Forma (Unaudited)	Year ended December 31,
	(in thousands, except per share data)	2013				2012
	Total revenue	$	2,257,098			$	2,149,391
	Income from continuing operations	$	302,354			$	634,457
	Net income attributable to Boston Properties, Inc.	$	400,017			$	626,174
	Basic earnings per share:
	Net income per share attributable to Boston Properties, Inc.	$	2.58			$	4.15
	Diluted earnings per share:
	Net income per share attributable to Boston Properties, Inc.	$	2.57			$	4.13
Summary Of The Discontinued Operations	The following table summarizes the income from discontinued operations related to One Preserve Parkway, 10 & 20 Burlington Mall Road, 1301 New York Avenue, 303 Almaden Boulevard, Montvale Center and Bedford Business Park and the related gains on sales of real estate, gain on forgiveness of debt and impairment loss for the years ended December 31, 2013 and 2012:
		For the year ended December 31,
		2013				2012
		(in thousands)
	Total revenue	$	20,138			$	32,607
	Expenses
	Operating	6,996				12,038
	Depreciation and amortization	4,760				8,169
	Total expenses	11,756				20,207
	Operating income	8,382				12,400
	Other expense
	Interest expense	360				2,594
	Income from discontinued operations	$	8,022			$	9,806
	Noncontrolling interest in income from discontinued operations – common units of the Operating Partnership	(803		)		(1,031		)
	Income from discontinued operations attributable to Boston Properties, Inc.	$	7,219			$	8,775
	Gains on sales of real estate from discontinued operations	$	112,829			$	36,877
	Gain on forgiveness of debt from discontinued operations	20,182				—
	Impairment loss from discontinued operations	(3,241		)		—
	Noncontrolling interest in gains on sales of real estate, gain on forgiveness of debt and impairment loss from discontinued operations – common units of the Operating Partnership	(13,348		)		(4,044		)
	Gains on sales of real estate, gain on forgiveness of debt and impairment loss from discontinued operations attributable to Boston Properties, Inc.	$	116,422			$	32,833
Mountain View Research And Technology Parks [Member]
Real Estate Properties [Line Items]
Schedule Of Allocation Of The Aggregate Purchase Price Of Acquisition
										Land	$	126,521
										Building and improvements	82,451
										Tenant improvements	7,326
										In-place lease intangibles	23,279
										Above-market rents	843
										Below-market rents	(7,336		)
										Net assets acquired	$	233,084
767 5th Avenue (The General Motors Building) [Member]
Real Estate Properties [Line Items]
Schedule Of Allocation Of The Aggregate Purchase Price Of Acquisition	On May 31, 2013, the Company's two joint venture partners in 767 Venture, LLC (the entity that owns 767 Fifth Avenue (the General Motors Building) located in New York City) transferred all of their interests in the joint venture to third parties. 767 Fifth Avenue (the General Motors Building) is a Class A office property totaling approximately 1.8 million net rentable square feet. In connection with the transfer, the Company and its new joint venture partners modified the Company's relative decision making authority and consent rights with respect to the joint venture's assets and operations. These changes resulted in the Company having sufficient financial and operating control over 767 Venture, LLC such that, effective as of May 31, 2013, the Company accounts for the assets, liabilities and operations of 767 Venture, LLC on a consolidated basis in its financial statements instead of under the equity method of accounting (See Note 11). The following table summarizes the allocation of the aggregate purchase price of 767 Fifth Avenue (the General Motors Building) at the date of consolidation on May 31, 2013 (in thousands) in accordance with the guidance in ASC 805 "Business Combinations."
	Real estate and related intangibles recorded upon consolidation
	Land	$	1,796,252
	Building and improvements	1,447,446
	Tenant improvements	85,208
	In-place lease intangibles	357,781
	Above market rents	101,897
	Below market rents	(239,641		)
	Above market assumed debt adjustments	(192,943		)
		$	3,356,000
	Debt recorded upon consolidation
	Mortgage notes payable	$	(1,300,000	)
	Mezzanine notes payable	(306,000		)
	Members' notes payable	(450,000		)	-1
		$	(2,056,000	)
	Working capital recorded upon consolidation
	Cash and cash equivalents	$	79,468
	Cash held in escrows	2,403
	Tenant and other receivables	7,104
	Prepaid expenses and other assets	4,269
	Accounts payable and accrued expenses	(2,418		)
	Accrued interest payable	(182,369		)	-2
	Other liabilities	(6,304		)
		$	(97,847	)
	Noncontrolling interest recorded upon consolidation
	Noncontrolling interests	$	(520,000	)
	Noncontrolling interests - working capital	39,139
		$	(480,861	)
	Net assets recorded upon consolidation	$	721,292
	_______________
	-1	The Company's member loan totaling $270.0 million eliminates in consolidation.
	-2	The Company's share of the accrued interest payable on the members' loans totaling approximately $105.5 million eliminates in consolidation.

Deferred_Charges_Tables

Deferred Charges (Tables)	12 Months Ended
	Dec. 31, 2014
Deferred Charges [Abstract]
Deferred Charges	Deferred charges consisted of the following at December 31, (in thousands):
		2014			2013
	Leasing costs, including lease related intangibles	$	1,234,192		$	1,183,204
	Financing costs	69,127			76,798
		1,303,319			1,260,002
	Less: Accumulated amortization	(471,575		)	(375,552		)
		$	831,744		$	884,450
Schedule of Finite-Lived Intangible Assets, Future Amortization Expense [Table Text Block]	The following table summarizes the scheduled amortization of the Company's acquired “above-” and “below-market” lease intangibles for each of the five succeeding years (in thousands). Accrued rental income as reported on the Consolidated Balance Sheets represents rental income recognized in excess of rent payments actually received pursuant to the terms of the individual lease agreements.
		Acquired Above-Market Lease Intangibles			Acquired Below-Market Lease Intangibles
	2015	$	22,671		$	57,019
	2016	20,491			51,460
	2017	12,277			35,896
	2018	8,637			33,215
	2019	7,106			27,615
	The following table summarizes the scheduled amortization of the Company's acquired in-place lease intangibles for each of the five succeeding years (in thousands).
		Acquired In-Place Lease Intangibles
	2015	$	66,390
	2016	55,327
	2017	38,812
	2018	33,964
	2019	27,439

Investments_in_Unconsolidated_1

Investments in Unconsolidated Joint Ventures (Tables)	12 Months Ended
	Dec. 31, 2014
Investments In Unconsolidated Joint Ventures [Abstract]
Investments In Unconsolidated Joint Ventures	The investments in unconsolidated joint ventures consist of the following at December 31, 2014:
	Entity		Properties		Nominal %				Carrying Value of Investment (1)
					Ownership
									(in thousands)
	Square 407 Limited Partnership		Market Square North		50	%			$	(8,022	)
	The Metropolitan Square Associates LLC		Metropolitan Square		51	%			8,539
	BP/CRF 901 New York Avenue LLC		901 New York Avenue		25	%		(2)	(1,080		)
	WP Project Developer LLC		Wisconsin Place Land and Infrastructure		33.3	%		(3)	45,514
	Annapolis Junction NFM, LLC		Annapolis Junction		50	%		-4	25,246
	540 Madison Venture LLC		540 Madison Avenue		60	%			68,128
	500 North Capitol LLC		500 North Capitol Street, NW		30	%			(2,250		)
	501 K Street LLC		1001 6th Street (formerly 501 K Street)		50	%		-5	41,736
	Podium Developer LLC		North Station (Phase I - Air Rights)		50	%			4,231
									$	182,042
	_______________
	-1	Investments with deficit balances aggregating approximately $11.4 million have been reflected within Other Liabilities on the Company's Consolidated Balance Sheets.
	-2	The Company’s economic ownership has increased based on the achievement of certain return thresholds.
	-3	The Company’s wholly-owned entity that owns the office component of the project also owns a 33.3% interest in the entity owning the land, parking garage and infrastructure of the project.
	-4	The joint venture owns two in-service buildings, two buildings under construction and two undeveloped land parcels.
	-5	Under the joint venture agreement, the partner will be entitled to up to two additional payments from the venture based on increases in total square footage of the project above 520,000 square feet and achieving certain project returns at stabilization.
Schedule Of Balance Sheets Of The Unconsolidated Joint Ventures [Text Block]	The combined summarized balance sheets of the Company's unconsolidated joint ventures are as follows:
		December 31,				December 31,
		2014				2013
		(in thousands)
	ASSETS
	Real estate and development in process, net	$	1,034,552			$	924,297
	Other assets	264,097				163,149
	Total assets	$	1,298,649			$	1,087,446
	LIABILITIES AND MEMBERS’/PARTNERS’ EQUITY
	Mortgage and notes payable	$	830,075			$	749,732
	Other liabilities	34,211				28,830
	Members’/Partners’ equity	434,363				308,884
	Total liabilities and members’/partners’ equity	$	1,298,649			$	1,087,446
	Company’s share of equity	$	209,828			$	154,726
	Basis differentials (1)	(27,786		)		(28,642		)
	Carrying value of the Company’s investments in unconsolidated joint ventures (2)	$	182,042			$	126,084
	_______________
	-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.
	-2	Investments with deficit balances aggregating approximately $11.4 million and $14.0 million at December 31, 2014 and 2013, respectively, have been reflected within Other Liabilities on the Company's Consolidated Balance Sheets.
Statements Of Operations Of The Joint Ventures	The combined summarized statements of operations of the Company's joint ventures are as follows:
		For the year ended December 31,
		2014				2013			2012
		(in thousands)
	Total revenue (1)	$	158,161			$	311,548		$	564,205
	Expenses
	Operating	62,974				105,319			162,665
	Depreciation and amortization	37,041				86,088			163,134
	Total expenses	100,015				191,407			325,799
	Operating income	58,146				120,141			238,406
	Other income (expense)
	Interest expense	(31,896		)		(112,535		)	(224,645		)
	Losses from early extinguishments of debt	—				(1,677		)	—
	Income from continuing operations	26,250				5,929			13,761
	Gains on sales of real estate	—				14,207			990
	Net income	$	26,250			$	20,136		$	14,751
	Company’s share of net income	$	11,913			$	4,612		$	6,863
	Gains on sales of real estate	—				54,501			—
	Basis differential	856				(1,017		)	1,732
	Elimination of inter-entity interest on partner loan	—				16,978			40,483
	Income from unconsolidated joint ventures	$	12,769			$	75,074		$	49,078
	Gains on consolidation of joint ventures	$	—			$	385,991		$	—
	_______________
	-1	Includes straight-line rent adjustments of $3.0 million, $7.8 million and $12.0 million for the years ended December 31, 2014, 2013 and 2012, respectively. Includes net above-/below-market rent adjustments of $(0.1) million, $33.7 million and $91.1 million for the years ended December 31, 2014, 2013 and 2012, respectively. Total revenue for the year ended December 31, 2012 includes termination income totaling approximately $19.6 million (of which the Company's share is approximately $11.8 million) related to a lease termination with a tenant at 767 Fifth Avenue (The General Motors Building).

Mortgage_Notes_Payable_Schedul

Mortgage Notes Payable Schedule of Aggregate Principal Payments On Mortgage Notes (Tables)	12 Months Ended
	Dec. 31, 2014
Mortgage Notes Payable [Abstract]
Schedule of Aggregate Principal Payments On Mortgage Notes [Table Text Block]	Contractual aggregate principal payments of mortgage notes payable at December 31, 2014 are as follows:
		Principal Payments
		(in thousands)
	2015	$	26,184
	2016	608,879
	2017	2,821,750
	2018	18,633
	2019	19,670
	Thereafter	675,657
	Total aggregate principal payments	4,170,773
	Unamortized balance of historical fair value adjustments	138,711
	Total carrying value of mortgage notes payable	$	4,309,484

Unsecured_Senior_Notes_Tables

Unsecured Senior Notes (Tables)	12 Months Ended
	Dec. 31, 2014
Debt Disclosure [Abstract]
Schedule of Senior Notes	The following summarizes the unsecured senior notes outstanding as of December 31, 2014 (dollars in thousands):
		Coupon/		Effective		Principal			Maturity Date(2)
		Stated Rate		Rate(1)		Amount
	10 Year Unsecured Senior Notes	5.875	%	5.967	%	$	700,000		October 15, 2019
	10 Year Unsecured Senior Notes	5.625	%	5.708	%	700,000			November 15, 2020
	10 Year Unsecured Senior Notes	4.125	%	4.289	%	850,000			May 15, 2021
	7 Year Unsecured Senior Notes	3.7	%	3.853	%	850,000			November 15, 2018
	11 Year Unsecured Senior Notes	3.85	%	3.954	%	1,000,000			February 1, 2023
	10.5 Year Unsecured Senior Notes	3.125	%	3.279	%	500,000			September 1, 2023
	10.5 Year Unsecured Senior Notes	3.8	%	3.916	%	700,000			February 1, 2024
	Total principal					5,300,000
	Net unamortized discount					(12,296		)
	Total					$	5,287,704
	_______________
	-1	Yield on issuance date including the effects of discounts on the notes and the amortization of financing costs.
	-2	No principal amounts are due prior to maturity.

Noncontrolling_Interests_Table

Noncontrolling Interests (Tables)	12 Months Ended
	Dec. 31, 2014
Noncontrolling Interest [Abstract]
Schedule of Redeemable Interest in Property Partnerships [Table Text Block]	The following table reflects the activity of the noncontrolling interest—redeemable interest in property partnership in the Company's Fountain Square consolidated joint venture for the years ended December 31, 2014, 2013 and 2012 (in thousands):
	Balance at December 31, 2011	$	—
	Acquisition-date fair value of redeemable interest	98,787
	Net loss	(719		)
	Distributions	(3,032		)
	Adjustment to reflect redeemable interest at redemption value	2,522
	Balance at December 31, 2012	97,558
	Net loss	(1,839		)
	Distributions	(4,585		)
	Adjustment to reflect redeemable interest at redemption value	8,475
	Balance at December 31, 2013	99,609
	Net loss	(603		)
	Distributions	(6,000		)
	Adjustment to reflect redeemable interest at redemption value	11,686			(1	)
	Balance at December 31, 2014	$	104,692
Schedule Of Redeemable Preferred Units of the Operating Partnership	The following table reflects the activity of the noncontrolling interests—redeemable preferred units of the Operating Partnership for the years ended December 31, 2014, 2013 and 2012 (in thousands):
	Balance at December 31, 2011	$	55,652
	Issuance of redeemable preferred units (Series Four Preferred Units)	79,405
	Net income	3,497
	Distributions	(3,497		)
	Redemption of redeemable preferred units (Series Four Preferred Units)	(18,329		)
	Conversion of redeemable preferred units (Series Two Preferred Units) to common units	(5,852		)
	Balance at December 31, 2012	110,876
	Net income	6,046
	Distributions	(6,046		)
	Redemption of redeemable preferred units (Series Four Preferred Units)	(43,070		)
	Conversion of redeemable preferred units (Series Two Preferred Units) to common units	(16,494		)
	Balance at December 31, 2013	51,312
	Net income	1,023
	Distributions	(1,023		)
	Redemption of redeemable preferred units (Series Four Preferred Units)	(17,373		)
	Conversion of redeemable preferred units (Series Two Preferred Units) to common units	(33,306		)
	Balance at December 31, 2014	$	633

Future_Minimum_Rents_Tables

Future Minimum Rents (Tables)	12 Months Ended
	Dec. 31, 2014
Future Minimum Rents [Abstract]
Schedule of Future Minimum Rental Payments for Operating Leases [Table Text Block]
	Years Ending December 31,	(in thousands)
	2015	$	1,796,517
	2016	1,788,978
	2017	1,677,005
	2018	1,568,200
	2019	1,483,188
	Thereafter	9,097,899

Segment_Information_Tables

Segment Information (Tables)	12 Months Ended
	Dec. 31, 2014
Segment Reporting [Abstract]
Schedule Of Segment Information By Geographic Area And Property Type	Information by geographic area and property type (dollars in thousands):
	For the year ended December 31, 2014:
		Boston				New York			San				Washington,			Total
									Francisco				DC
	Rental Revenue:
	Class A Office	$	692,116			$	928,692		$	237,381			$	381,930		$	2,240,119
	Office/Technical	23,801				—			23,840				14,344			61,985
	Residential	4,528				—			—				21,665			26,193
	Hotel	43,385				—			—				—			43,385
	Total	763,830				928,692			261,221				417,939			2,371,682
	% of Grand Totals	32.21		%		39.16		%	11.01		%		17.62		%	100		%
	Rental Expenses:
	Class A Office	270,947				315,330			85,178				131,447			802,902
	Office/Technical	7,173				—			4,955				4,338			16,466
	Residential	1,957				—			—				13,965			15,922
	Hotel	29,236				—			—				—			29,236
	Total	309,313				315,330			90,133				149,750			864,526
	% of Grand Totals	35.78		%		36.47		%	10.43		%		17.32		%	100		%
	Net operating income	$	454,517			$	613,362		$	171,088			$	268,189		$	1,507,156
	% of Grand Totals	30.16		%		40.7		%	11.35		%		17.79		%	100		%
	For the year ended December 31, 2013:
		Boston				New York			San				Washington,			Total
									Francisco				DC
	Rental Revenue:
	Class A Office	$	665,991			$	725,566		$	214,755			$	381,359		$	1,987,671
	Office/Technical	22,617				—			17,259				15,649			55,525
	Residential	4,395				—			—				17,923			22,318
	Hotel	40,330				—			—				—			40,330
	Total	733,333				725,566			232,014				414,931			2,105,844
	% of Grand Totals	34.82		%		34.46		%	11.02		%		19.7		%	100		%
	Rental Expenses:
	Class A Office	259,997				251,640			77,905				126,507			716,049
	Office/Technical	6,879				—			3,708				4,190			14,777
	Residential	1,823				—			—				10,307			12,130
	Hotel	28,447				—			—				—			28,447
	Total	297,146				251,640			81,613				141,004			771,403
	% of Grand Totals	38.52		%		32.62		%	10.58		%		18.28		%	100		%
	Net operating income	$	436,187			$	473,926		$	150,401			$	273,927		$	1,334,441
	% of Grand Totals	32.69		%		35.51		%	11.27		%		20.53		%	100		%
	For the year ended December 31, 2012:
		Boston				New York			San				Washington,			Total
									Francisco				DC
	Rental Revenue:
	Class A Office	$	617,652			$	543,194		$	208,177			$	346,402		$	1,715,425
	Office/Technical	22,460				—			494				16,264			39,218
	Residential	3,936				—			—				16,632			20,568
	Hotel	37,915				—			—				—			37,915
	Total	681,963				543,194			208,671				379,298			1,813,126
	% of Grand Totals	37.61		%		29.96		%	11.51		%		20.92		%	100		%
	Rental Expenses:
	Class A Office	242,904				187,987			75,542				111,049			617,482
	Office/Technical	6,499				—			149				3,966			10,614
	Residential	1,675				—			—				9,317			10,992
	Hotel	28,120				—			—				—			28,120
	Total	279,198				187,987			75,691				124,332			667,208
	% of Grand Totals	41.85		%		28.18		%	11.34		%		18.63		%	100		%
	Net operating income	$	402,765			$	355,207		$	132,980			$	254,966		$	1,145,918
	% of Grand Totals	35.15		%		31		%	11.6		%		22.25		%	100		%
Schedule Of Reconciliation Of Net Operating Income To Net Income	The following is a reconciliation of Net Operating Income to net income attributable to Boston Properties, Inc. (in thousands):
		Year ended December 31,
			2014				2013			2012
	Net Operating Income		$	1,507,156			$	1,334,441		$	1,145,918
	Add:
	Development and management services income		25,316				29,695			34,060
	Income from unconsolidated joint ventures		12,769				75,074			49,078
	Gains on consolidation of joint ventures		—				385,991			—
	Interest and other income		8,765				8,310			10,091
	Gains from investments in securities		1,038				2,911			1,389
	Gains (losses) from early extinguishments of debt		(10,633		)		122			(4,453		)
	Income from discontinued operations		—				8,022			9,806
	Gains on sales of real estate from discontinued operations		—				112,829			36,877
	Gain on forgiveness of debt from discontinued operations		—				20,182			—
	Gains on sales of real estate		168,039				—			—
	Less:
	General and administrative expense		98,937				115,329			90,129
	Transaction costs		3,140				1,744			3,653
	Depreciation and amortization expense		628,573				560,637			445,875
	Interest expense		455,743				446,880			410,970
	Impairment loss		—				8,306			—
	Impairment loss from discontinued operations		—				3,241			—
	Noncontrolling interest in property partnerships		30,561				1,347			3,792
	Noncontrolling interest—redeemable preferred units of the Operating Partnership		1,023				6,046			3,497
	Noncontrolling interest—common units of the Operating Partnership		50,862				70,085			30,125
	Noncontrolling interest in discontinued operations—common units of the Operating Partnership		—				14,151			5,075
	Preferred dividends		10,500				8,057			—
	Net income attributable to Boston Properties, Inc. common shareholders		$	433,111			$	741,754		$	289,650

Earnings_Per_Share_Tables

Earnings Per Share (Tables)	12 Months Ended
	Dec. 31, 2014
Earnings Per Share [Abstract]
Computation Of Basic And Diluted Earnings Per Share
		For the Year Ended December 31, 2014
		Income			Shares	Per Share
		(Numerator)			(Denominator)	Amount
		(in thousands, except for per share amounts)
	Basic Earnings:
	Net income attributable to Boston Properties, Inc. common shareholders	$	433,111		153,089	$	2.83
	Effect of Dilutive Securities:
	Stock Based Compensation	—			219	—
	Diluted Earnings:
	Net income attributable to Boston Properties, Inc. common shareholders	$	433,111		153,308	$	2.83
		For the Year Ended December 31, 2013
		Income			Shares	Per Share
		(Numerator)			(Denominator)	Amount
		(in thousands, except for per share amounts)
	Basic Earnings:
	Income from continuing operations attributable to Boston Properties, Inc.	$	618,113		152,201	$	4.06
	Discontinued operations attributable to Boston Properties, Inc.	123,641			—	0.81
	Allocation of undistributed earnings to participating securities	(160		)	—	—
	Net income attributable to Boston Properties, Inc. common shareholders	$	741,594		152,201	$	4.87
	Effect of Dilutive Securities:
	Stock Based Compensation and Exchangeable Senior Notes	—			320	(0.01		)
	Diluted Earnings:
	Net income attributable to Boston Properties, Inc. common shareholders	$	741,594		152,521	$	4.86
		For the Year Ended December 31, 2012
		Income			Shares	Per Share
		(Numerator)			(Denominator)	Amount
		(in thousands, except for per share amounts)
	Basic Earnings:
	Income from continuing operations attributable to Boston Properties, Inc.	$	248,042		150,120	$	1.65
	Discontinued operations attributable to Boston Properties, Inc.	41,608			—	0.28
	Net income attributable to Boston Properties, Inc. common shareholders	$	289,650		150,120	$	1.93
	Effect of Dilutive Securities:
	Stock Based Compensation and Exchangeable Senior Notes	—			591	(0.01		)
	Diluted Earnings:
	Net income attributable to Boston Properties, Inc. common shareholders	$	289,650		150,711	$	1.92

Stock_Option_and_Incentive_Pla1

Stock Option and Incentive Plan Stock Option and Incentive Plan and Stock Purchase Plan (Tables)	12 Months Ended
	Dec. 31, 2014
Stock Option and Incentive Plan and Stock Purchase Plan [Abstract]
Schedule of Share-based Compensation, Stock Options, Activity [Table Text Block]
		Shares		Weighted
				Average
				Exercise
				Price
	Outstanding at December 31, 2011	155,623		$	89.35
	Granted	186,007		$	107.23
	Exercised	(22,823	)	$	72.42
	Canceled	(24,280	)	$	100.15
	Outstanding at December 31, 2012	294,527		$	101.06
	Granted	252,220		$	104.5
	Special dividend adjustment	12,076		$	100.44
	Outstanding at December 31, 2013	558,823		$	100.43
	Exercised	(21,459	)	$	97.04
	Canceled	(2,444	)	$	103.57
	Special dividend adjustment	18,392		$	97.22
	Outstanding at December 31, 2014	553,312		$	97.21
Schedule of Stock Options Outstanding [Table Text Block]	The following table summarizes information about stock options outstanding at December 31, 2014:
	Options Outstanding					Options Exercisable
	Number		Weighted-Average Remaining	Exercise Price		Number Exercisable	Exercise Price
	Outstanding at		Contractual Life			at 12/31/14
	12/31/14
	124,513		3.6 years	$	87.7	111,832	$	87.7
	53,759		8.3 years	$	96.62	13,439	$	96.62
	207,064		4.9 years	$	99.41	150,024	$	99.41
	167,976		4.4 years	$	101.75	135,848	$	101.75

Selected_Interim_Financial_Inf1

Selected Interim Financial Information (unaudited) (Tables)	12 Months Ended
	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Schedule of Quarterly Financial Information [Table Text Block]
		2014 Quarter Ended
		March 31,		June 30,		September 30,		December 31,
		(in thousands, except for per share amounts)
	Total revenue	$	574,694	$	589,794	$	618,803	$	613,707
	Income from continuing operations	$	67,756	$	95,901	$	109,038	$	85,323
	Net income attributable to Boston Properties, Inc. common sharedholders	$	54,034	$	76,527	$	127,724	$	174,510
	Income attributable to Boston Properties, Inc. per share—basic	$	0.35	$	0.5	$	0.83	$	1.14
	Income attributable to Boston Properties, Inc. per share—diluted	$	0.35	$	0.5	$	0.83	$	1.14
		2013 Quarter Ended
		March 31,		June 30,		September 30,		December 31,
		(in thousands, except for per share amounts)
	Total revenue	$	477,826	$	510,033	$	571,481	$	576,199
	Income from continuing operations	$	38,496	$	503,446	$	82,311	$	79,395
	Net income attributable to Boston Properties, Inc. common sharedholders	$	47,854	$	452,417	$	152,677	$	88,719
	Income attributable to Boston Properties, Inc. per share—basic	$	0.32	$	2.95	$	1	$	0.58
	Income attributable to Boston Properties, Inc. per share—diluted	$	0.31	$	2.94	$	1	$	0.58

Real_Estate_and_Accumulated_De1

Real Estate and Accumulated Depreciation Real estate and accumulated depreciation (Tables)	12 Months Ended
	Dec. 31, 2014
SEC Schedule III, Real Estate and Accumulated Depreciation Disclosure [Abstract]
US SEC Schedule III, Reconciliation of Real Estate Accumulated Depreciation [Table Text Block]	Boston Properties, Inc.
	Schedule 3 - Real Estate and Accumulated Depreciation
	December 31, 2014
	(dollars in thousands)
	Property Name		Type		Location	Encumbrances					Original				Costs			Land and Improvements		Building and		Land		Development		Total		Accumulated		Year(s) Built/	Depreciable
															Capitalized					Improvements		Held for		and				Depreciation		Renovated	Lives  (Years)
															Subsequent							Development		Construction
															to									in Progress
											Land		Building		Acquisition
	767 Fifth Avenue (The General Motors Building)		Office		New York, NY	$	1,421,083				$	1,796,252	$	1,532,654	$	19,627		$	1,796,252	$	1,552,281	$	—	$	—	$	3,348,533	$	80,075	1968	-1
	Embarcadero Center		Office		San Francisco, CA	354,680					179,697		847,410		317,320			195,987		1,148,440		—		—		1,344,427		505,702		1970/1989	-1
	Prudential Center		Office		Boston, MA	—					92,077		734,594		380,216			107,426		1,064,013		404		35,044		1,206,887		421,371		1965/1993/2002	-1
	399 Park Avenue		Office		New York, NY	—					339,200		700,358		109,630			354,107		795,081		—		—		1,149,188		242,572		1961	-1
	601 Lexington Avenue		Office		New York, NY	710,932					241,600		494,782		230,137			289,639		676,880		—		—		966,519		228,397		1977/1997	-1
	The John Hancock Tower and Garage		Office		Boston, MA	649,108					219,543		667,884		74,060			219,616		740,863		1,008		—		961,487		104,888		1976	-1
	250 West 55th Street		Office		New York, NY	—					285,263		603,167		—			285,263		603,167		—		—		888,430		9,240		2014	-1
	Times Square Tower		Office		New York, NY	—					165,413		380,438		84,488			169,193		461,146		—		—		630,339		150,619		2004	-1
	100 Federal Street		Office		Boston, MA	—					131,067		435,954		9,711			131,067		445,432		233		—		576,732		44,931		1971-1975	-1
	Carnegie Center		Office		Princeton, NJ	—					105,107		377,259		83,053			103,064		450,653		2,350		9,352		565,419		183,866		1983-1999	-1
	Atlantic Wharf		Office		Boston, MA	—					63,988		454,537		15,169			63,988		469,706		—		—		533,694		55,070		2011	-1
	Fountain Square		Office		Reston, VA	220,133					56,853		306,298		6,938			56,853		313,236		—		—		370,089		27,645		1986-1990	-1
	510 Madison Avenue		Office		New York, NY	—					103,000		253,665		12,912			103,000		266,577		—		—		369,577		24,871		2012	-1
	599 Lexington Avenue		Office		New York, NY	750,000					81,040		100,507		128,649			87,852		222,344		—		—		310,196		152,958		1986	-1
	680 Folsom Street		Office		San Francisco, CA	—					106,510		185,801		—			106,510		185,801		—		—		292,311		4,161		2014	-1
	South of Market and Democracy Tower		Office		Reston, VA	—					13,603		237,479		9,790			13,687		247,185		—		—		260,872		60,668		2008-2009	-1
	Bay Colony Corporate Center		Office		Waltham, MA	—					18,789		148,451		58,126			18,789		206,577		—		—		225,366		31,089		1985-1989	-1
	Gateway Center		Office		San Francisco, CA	—					28,255		139,245		52,768			30,627		189,641		—		—		220,268		87,759		1984/1986/2002	-1
	2200 Pennsylvania Avenue		Office		Washington, DC	—					—		183,541		5,141			—		188,682		—		—		188,682		26,612		2011	-1
	Mountain View Research Park		Office		Mountain View, CA	—					95,066		68,373		3,267			95,066		71,640		—		—		166,706		6,044		1977-1981/2007-2013	-1
	3100-3130 Zanker Road (formerly 3200 Zanker Road)		Office		San Jose, CA	—					36,705		82,863		28,919			36,997		107,737		3,753		—		148,487		25,914		1988	-1
	Reservoir Place		Office		Waltham, MA	—					18,605		92,619		34,732			20,118		125,830		8		—		145,956		57,456		1955/1987	-1
	1333 New Hampshire Avenue		Office		Washington, DC	—					34,032		85,660		9,947			35,382		94,257		—		—		129,639		32,942		1996	-1
	505 9th Street		Office		Washington, DC	118,919					38,885		83,719		5,692			42,082		86,214		—		—		128,296		23,366		2007	-1
	Kingstowne Towne Center		Office		Alexandria, VA	31,364					18,021		109,038		1,013			18,062		110,010		—		—		128,072		29,821		2003-2006	-1
	1330 Connecticut Avenue		Office		Washington, DC	—					25,982		82,311		16,379			27,135		97,537		—		—		124,672		32,742		1984	-1
	Capital Gallery		Office		Washington, DC	—					4,725		29,565		88,078			8,662		113,706		—		—		122,368		53,320		1981/2006	-1
	Weston Corporate Center		Office		Weston, MA	—					25,753		92,312		(123		)	25,854		92,088		—		—		117,942		13,972		2010	-1
	One Freedom Square		Office		Reston, VA	—					9,929		84,504		18,898			11,293		102,038		—		—		113,331		36,912		2000	-1
	Two Freedom Square		Office		Reston, VA	—					13,930		77,739		19,563			15,420		95,812		—		—		111,232		38,275		2001	-1
	One and Two Reston Overlook		Office		Reston, VA	—					16,456		66,192		25,320			16,179		91,789		—		—		107,968		35,274		1999	-1
	415 Main Street (formerly Seven Cambridge Center)		Office		Cambridge, MA	—					3,457		97,136		2,880			4,125		99,348		—		—		103,473		53,469		2006	-1
	Discovery Square		Office		Reston, VA	—					11,198		71,782		19,977			12,533		90,424		—		—		102,957		32,326		2001	-1
	140 Kendrick Street		Office		Needham, MA	—					18,095		66,905		14,870			19,092		80,778		—		—		99,870		20,193		2000	-1
	355 Main Street (formerly Five Cambridge Center)		Office		Cambridge, MA	—					18,863		53,346		25,899			21,173		76,935		—		—		98,108		20,864		1981/1996/2013	-1
	90 Broadway (formerly Four Cambridge Center)		Office		Cambridge, MA	—					19,104		52,078		13,680			20,785		64,077		—		—		84,862		12,426		1983/1998/2013	-1
	Waltham Weston Corporate Center		Office		Waltham, MA	—					10,385		60,694		8,105			11,097		68,087		—		—		79,184		22,151		2003	-1
	230 CityPoint		Office		Waltham, MA	—					13,189		49,823		15,744			13,593		65,163		—		—		78,756		19,029		1992	-1
	77 CityPoint		Office		Waltham, MA	—					13,847		60,383		3,163			13,873		63,520		—		—		77,393		14,639		2008	-1
	North First Business Park		Office		San Jose, CA	—					58,402		13,069		4,028			23,377		16,214		35,908		—		75,499		12,174		1981	-1
	300 Binney Street (formerly Seventeen Cambridge Center)		Office		Cambridge, MA	—					18,080		51,262		165			18,080		51,427		—		—		69,507		2,938		2013	-1
	2440 West El Camino Real		Office		Mountain View, CA	—					16,741		51,285		716			16,741		52,001		—		—		68,742		6,400		1987/2003	-1
	Wisconsin Place		Office		Chevy Chase, MD	—					—		53,349		12,787			—		66,136		—		—		66,136		14,300		2009	-1
	Reston Corporate Center		Office		Reston, VA	—					9,135		50,857		5,361			10,148		55,205		—		—		65,353		22,540		1984	-1
	New Dominion Technology Park, Bldg. Two		Office		Herndon, VA	—					5,584		51,868		3,911			6,510		54,853		—		—		61,363		18,312		2004	-1
	Sumner Square		Office		Washington, DC	—					624		28,745		25,004			1,478		52,895		—		—		54,373		25,152		1985	-1
	New Dominion Technology Park, Bldg. One		Office		Herndon, VA	40,975					3,880		43,227		3,883			4,583		46,407		—		—		50,990		21,158		2001	-1
	200 West Street		Office		Waltham, MA	—					16,148		24,983		9,782			16,813		34,100		—		—		50,913		16,439		1999	-1
	191 Spring Street		Office		Lexington, MA	—					2,850		27,166		18,558			3,151		45,423		—		—		48,574		31,087		1971/1995	-1
	255 Main Street (formerly One Cambridge Center)		Office		Cambridge, MA	—					134		25,110		20,596			548		45,292		—		—		45,840		27,168		1987	-1
	University Place		Office		Cambridge, MA	12,290					—		37,091		6,794			390		43,495		—		—		43,885		22,156		1985	-1
	2600 Tower Oaks Boulevard		Office		Rockville, MD	—					4,243		31,125		7,924			4,785		38,507		—		—		43,292		16,993		2001	-1
	Quorum Office Park		Office		Chelmsford, MA	—					3,750		32,454		5,814			5,187		36,831		—		—		42,018		13,710		2001	-1
	500 E Street		Office		Washington, DC	—					109		22,420		13,020			2,379		33,170		—		—		35,549		21,601		1987	-1
	150 Broadway (formerly Eight Cambridge Center)		Office		Cambridge, MA	—					850		25,042		8,507			1,323		33,076		—		—		34,399		12,606		1999	-1
325 Main Street (formerly Three Cambridge Center)		Office		Cambridge, MA	—					174		12,200		11,774			965		23,183		—		—		24,148		10,496		1987/2013	-1
105 Broadway (formerly Ten Cambridge Center)		Office		Cambridge, MA	—					1,299		12,943		6,851			2,395		18,698		—		—		21,093		11,948		1990	-1
40 Shattuck Road		Office		Andover, MA	—					709		14,740		2,697			893		17,253		—		—		18,146		6,171		2001	-1
201 Spring Street		Office		Lexington, MA	—					2,849		15,303		(154		)	3,124		14,874		—		—		17,998		6,511		1997	-1
Lexington Office Park		Office		Lexington, MA	—					998		1,426		13,941			1,264		15,101		—		—		16,365		11,346		1982	-1
92-100 Hayden Avenue		Office		Lexington, MA	—					594		6,748		7,819			802		14,359		—		—		15,161		10,665		1985	-1
91 Hartwell Avenue		Office		Lexington, MA	—					784		6,464		6,747			941		13,054		—		—		13,995		7,655		1985	-1
181 Spring Street		Office		Lexington, MA	—					1,066		9,520		1,950			1,160		11,376		—		—		12,536		4,306		1999	-1
195 West Street		Office		Waltham, MA	—					1,611		6,652		4,267			1,858		10,672		—		—		12,530		6,626		1990	-1
33 Hayden Avenue		Office		Lexington, MA	—					266		3,234		8,787			425		11,862		—		—		12,287		5,558		1979	-1
145 Broadway (formerly Eleven Cambridge Center)		Office		Cambridge, MA	—					121		5,535		5,278			324		10,610		—		—		10,934		7,566		1984	-1
7501 Boston Boulevard, Building Seven		Office		Springfield, VA	—					665		9,273		520			791		9,667		—		—		10,458		4,113		1997	-1
7435 Boston Boulevard, Building One		Office		Springfield, VA	—					392		3,822		3,798			659		7,353		—		—		8,012		5,794		1982	-1
7450 Boston Boulevard, Building Three		Office		Springfield, VA	—					1,165		4,681		1,880			1,430		6,296		—		—		7,726		3,056		1987	-1
8000 Grainger Court, Building Five		Office		Springfield, VA	—					366		4,282		2,684			601		6,731		—		—		7,332		5,142		1984	-1
453 Ravendale Drive		Office		Mountain View, CA	—					5,477		1,090		230			5,477		1,320		—		—		6,797		213		1977	-1
7500 Boston Boulevard, Building Six		Office		Springfield, VA	—					138		3,749		2,538			406		6,019		—		—		6,425		4,265		1985	-1
7601 Boston Boulevard, Building Eight		Office		Springfield, VA	—					200		878		4,826			551		5,353		—		—		5,904		3,688		1986	-1
250 Binney Street (formerly Fourteen Cambridge Center)		Office		Cambridge, MA	—					110		4,483		1,223			273		5,543		—		—		5,816		3,943		1983	-1
7300 Boston Boulevard, Building Thirteen		Office		Springfield, VA	—					608		4,773		230			661		4,950		—		—		5,611		4,229		2002	-1
8000 Corporate Court, Building Eleven		Office		Springfield, VA	—					136		3,071		1,611			775		4,043		—		—		4,818		2,494		1989	-1
7375 Boston Boulevard, Building Ten		Office		Springfield, VA	—					23		2,685		1,003			93		3,618		—		—		3,711		2,362		1988	-1
7374 Boston Boulevard, Building Four		Office		Springfield, VA	—					241		1,605		1,729			398		3,177		—		—		3,575		2,235		1984	-1
7451 Boston Boulevard, Building Two		Office		Springfield, VA	—					249		1,542		1,661			613		2,839		—		—		3,452		2,135		1982	-1
32 Hartwell Avenue		Office		Lexington, MA	—					168		1,943		809			314		2,606		—		—		2,920		1,685		1968-1979/1987	-1
164 Lexington Road		Office		Billerica, MA	—					592		1,370		319			643		1,638		—		—		2,281		745		1982	-1
17 Hartwell Avenue		Office		Lexington, MA	—					26		150		1,018			65		1,113		16		—		1,194		714		1968	-1
Residences on The Avenue,		Residential		Washington, DC	—					—		119,874		(28		)	—		119,846		—		—		119,846		11,039		2011	-1
2221 I St., NW
The Avant at Reston Town Center		Residential		Reston, VA	—					20,350		91,995		—			20,350		91,995		—		—		112,345		2,397		2014	-1
The Lofts at Atlantic Wharf		Residential		Boston, MA	—					3,529		54,891		1,543			3,529		56,434		—		—		59,963		5,093		2011	-1
Boston Marriott Cambridge (formerly Cambridge Center Marriott)		Hotel		Cambridge, MA	—					478		37,918		36,160			1,201		73,355		—		—		74,556		47,196		1986	-1
Cambridge Center East Garage		Garage		Cambridge, MA	—					—		35,035		1,561			103		36,493		—		—		36,596		7,647		1984	-1
Cambridge Center West Garage		Garage		Cambridge, MA	—					1,256		15,697		1,571			1,434		17,090		—		—		18,524		3,815		2006	-1
Cambridge Center North Garage		Garage		Cambridge, MA	—					1,163		11,633		2,770			1,579		13,987		—		—		15,566		8,531		1990	-1
Salesforce Tower		Development		San Francisco, CA	—					—		—		345,303			—		—		—		345,303		345,303		—		N/A	N/A
601 Massachusetts Avenue		Development		Washington, DC	—					—		—		189,173			12		—		—		189,161		189,173		—		N/A	N/A
535 Mission Street		Development		San Francisco, CA	—					—		—		171,096			10,789		39,307		—		121,000		171,096		225		N/A	N/A
10 CityPoint		Development		Waltham, MA	—					—		—		21,533			163		—		—		21,370		21,533		—		N/A	N/A
690 Folsom Street		Development		San Francisco, CA	—					—		—		13,237			1,777		6,546		—		4,914		13,237		11		N/A	N/A
Springfield Metro Center		Land		Springfield, VA	—					—		—		32,445			—		—		32,445		—		32,445		—		N/A	N/A
Reston Signature Site		Land		Reston, VA	—					—		—		30,256			—		—		30,256		—		30,256		—		N/A	N/A
Plaza at Almaden		Land		San Jose, CA	—					—		—		29,006			—		—		29,006		—		29,006		—		N/A	N/A
Tower Oaks Master Plan		Land		Rockville, MD	—					—		—		28,919			—		—		28,919		—		28,919		—		N/A	N/A
214 Third Avenue (formerly Prospect Hill)		Land		Waltham, MA	—					—		—		23,255			—		132		12,956		10,167		23,255		—		N/A	N/A
Washingtonian North		Land		Gaithersburg, MD	—					—		—		18,813			—		—		18,813		—		18,813		—		N/A	N/A
6601 & 6605 Springfield Center Drive		Land		Springfield, VA	—					—		—		13,866			—		—		13,866		—		13,866		—		N/A	N/A
103 Fourth Avenue		Land		Waltham, MA	—					—		—		11,920			—		—		11,920		—		11,920		—		N/A	N/A
Reston Gateway		Land		Reston, VA	—					—		—		9,933			—		—		9,933		—		9,933		—		N/A	N/A
Reston Eastgate		Land		Reston, VA	—					—		—		8,817			—		—		8,817		—		8,817		—		N/A	N/A
Crane Meadow		Land		Marlborough, MA	—					—		—		8,726			—		—		8,726		—		8,726		—		N/A	N/A
Broad Run Business Park		Land		Loudoun County, VA	—					—		—		6,311			—		—		6,311		—		6,311		—		N/A	N/A
20 CityPoint		Land		Waltham, MA	—					—		—		4,801			—		—		4,801		—		4,801		—		N/A	N/A
Cambridge Master Plan		Land		Cambridge, MA	—					—		—		3,527			—		—		3,527		—		3,527		—		N/A	N/A
North First Master Plan		Land		San Jose, CA	—					—		—		1,664			—		—		1,664		—		1,664		—		N/A	N/A
425 Fourth Street		Land		San Francisco, CA	—					—		—		1,261			—		—		1,261		—		1,261		—		N/A	N/A
30 Shattuck Road		Land		Andover, MA	—					—		—		1,213			—		—		1,213		—		1,213		—		N/A	N/A
					$	4,309,484		(2	)	$	4,661,817	$	11,331,324	$	3,215,276		$	4,785,772	$	13,418,220	$	268,114	$	736,311	$	19,208,417	$	3,529,978
Note: Total Real Estate does not include Furniture, Fixtures and Equipment totaling approximately $27,986. Accumulated Depreciation does not include approximately $17,681 of accumulated depreciation related to Furniture, Fixtures and Equipment.
The aggregate cost and accumulated depreciation for tax purposes was approximately $14.3 billion and $2.6 billion, respectively.
-1	Depreciation of the buildings and improvements are calculated over lives ranging from the life of the lease to 40 years.
-2	Includes the unamortized balance of the historical fair value adjustment totaling approximately $138.7 million.
A summary of activity for real estate and accumulated depreciation is as follows:
		2014			2013				2012
Real Estate:
Balance at the beginning of the year		$	18,953,601		$	14,869,887			$	13,363,113
Additions to/improvements of real estate		594,296			4,410,622				1,602,583
Assets sold/written-off		(339,480		)	(326,908		)		(95,809		)
Balance at the end of the year		$	19,208,417		$	18,953,601			$	14,869,887
Accumulated Depreciation:
Balance at the beginning of the year		$	3,145,701		$	2,919,479			$	2,626,324
Depreciation expense		456,176			419,908				367,625
Assets sold/written-off		(71,899		)	(193,686		)		(74,470		)
Balance at the end of the year		$	3,529,978		$	3,145,701			$	2,919,479
Note: Real Estate and Accumulated Depreciation amounts do not include Furniture, Fixtures and Equipment.

Real_Estate_and_Accumulatied_D

Real Estate and Accumulatied Deprecitation Reconcilation Real estate and accumulated depreciation recocniliation (Tables)	12 Months Ended
	Dec. 31, 2014
Reconciliation of Real Estate and Accumulated Depreciation [Abstract]
US SEC Schedule III, Reconciliation of Real Estate Accumulated Depreciation [Table Text Block]	Boston Properties, Inc.
	Schedule 3 - Real Estate and Accumulated Depreciation
	December 31, 2014
	(dollars in thousands)
	Property Name		Type		Location	Encumbrances					Original				Costs			Land and Improvements		Building and		Land		Development		Total		Accumulated		Year(s) Built/	Depreciable
															Capitalized					Improvements		Held for		and				Depreciation		Renovated	Lives  (Years)
															Subsequent							Development		Construction
															to									in Progress
											Land		Building		Acquisition
	767 Fifth Avenue (The General Motors Building)		Office		New York, NY	$	1,421,083				$	1,796,252	$	1,532,654	$	19,627		$	1,796,252	$	1,552,281	$	—	$	—	$	3,348,533	$	80,075	1968	-1
	Embarcadero Center		Office		San Francisco, CA	354,680					179,697		847,410		317,320			195,987		1,148,440		—		—		1,344,427		505,702		1970/1989	-1
	Prudential Center		Office		Boston, MA	—					92,077		734,594		380,216			107,426		1,064,013		404		35,044		1,206,887		421,371		1965/1993/2002	-1
	399 Park Avenue		Office		New York, NY	—					339,200		700,358		109,630			354,107		795,081		—		—		1,149,188		242,572		1961	-1
	601 Lexington Avenue		Office		New York, NY	710,932					241,600		494,782		230,137			289,639		676,880		—		—		966,519		228,397		1977/1997	-1
	The John Hancock Tower and Garage		Office		Boston, MA	649,108					219,543		667,884		74,060			219,616		740,863		1,008		—		961,487		104,888		1976	-1
	250 West 55th Street		Office		New York, NY	—					285,263		603,167		—			285,263		603,167		—		—		888,430		9,240		2014	-1
	Times Square Tower		Office		New York, NY	—					165,413		380,438		84,488			169,193		461,146		—		—		630,339		150,619		2004	-1
	100 Federal Street		Office		Boston, MA	—					131,067		435,954		9,711			131,067		445,432		233		—		576,732		44,931		1971-1975	-1
	Carnegie Center		Office		Princeton, NJ	—					105,107		377,259		83,053			103,064		450,653		2,350		9,352		565,419		183,866		1983-1999	-1
	Atlantic Wharf		Office		Boston, MA	—					63,988		454,537		15,169			63,988		469,706		—		—		533,694		55,070		2011	-1
	Fountain Square		Office		Reston, VA	220,133					56,853		306,298		6,938			56,853		313,236		—		—		370,089		27,645		1986-1990	-1
	510 Madison Avenue		Office		New York, NY	—					103,000		253,665		12,912			103,000		266,577		—		—		369,577		24,871		2012	-1
	599 Lexington Avenue		Office		New York, NY	750,000					81,040		100,507		128,649			87,852		222,344		—		—		310,196		152,958		1986	-1
	680 Folsom Street		Office		San Francisco, CA	—					106,510		185,801		—			106,510		185,801		—		—		292,311		4,161		2014	-1
	South of Market and Democracy Tower		Office		Reston, VA	—					13,603		237,479		9,790			13,687		247,185		—		—		260,872		60,668		2008-2009	-1
	Bay Colony Corporate Center		Office		Waltham, MA	—					18,789		148,451		58,126			18,789		206,577		—		—		225,366		31,089		1985-1989	-1
	Gateway Center		Office		San Francisco, CA	—					28,255		139,245		52,768			30,627		189,641		—		—		220,268		87,759		1984/1986/2002	-1
	2200 Pennsylvania Avenue		Office		Washington, DC	—					—		183,541		5,141			—		188,682		—		—		188,682		26,612		2011	-1
	Mountain View Research Park		Office		Mountain View, CA	—					95,066		68,373		3,267			95,066		71,640		—		—		166,706		6,044		1977-1981/2007-2013	-1
	3100-3130 Zanker Road (formerly 3200 Zanker Road)		Office		San Jose, CA	—					36,705		82,863		28,919			36,997		107,737		3,753		—		148,487		25,914		1988	-1
	Reservoir Place		Office		Waltham, MA	—					18,605		92,619		34,732			20,118		125,830		8		—		145,956		57,456		1955/1987	-1
	1333 New Hampshire Avenue		Office		Washington, DC	—					34,032		85,660		9,947			35,382		94,257		—		—		129,639		32,942		1996	-1
	505 9th Street		Office		Washington, DC	118,919					38,885		83,719		5,692			42,082		86,214		—		—		128,296		23,366		2007	-1
	Kingstowne Towne Center		Office		Alexandria, VA	31,364					18,021		109,038		1,013			18,062		110,010		—		—		128,072		29,821		2003-2006	-1
	1330 Connecticut Avenue		Office		Washington, DC	—					25,982		82,311		16,379			27,135		97,537		—		—		124,672		32,742		1984	-1
	Capital Gallery		Office		Washington, DC	—					4,725		29,565		88,078			8,662		113,706		—		—		122,368		53,320		1981/2006	-1
	Weston Corporate Center		Office		Weston, MA	—					25,753		92,312		(123		)	25,854		92,088		—		—		117,942		13,972		2010	-1
	One Freedom Square		Office		Reston, VA	—					9,929		84,504		18,898			11,293		102,038		—		—		113,331		36,912		2000	-1
	Two Freedom Square		Office		Reston, VA	—					13,930		77,739		19,563			15,420		95,812		—		—		111,232		38,275		2001	-1
	One and Two Reston Overlook		Office		Reston, VA	—					16,456		66,192		25,320			16,179		91,789		—		—		107,968		35,274		1999	-1
	415 Main Street (formerly Seven Cambridge Center)		Office		Cambridge, MA	—					3,457		97,136		2,880			4,125		99,348		—		—		103,473		53,469		2006	-1
	Discovery Square		Office		Reston, VA	—					11,198		71,782		19,977			12,533		90,424		—		—		102,957		32,326		2001	-1
	140 Kendrick Street		Office		Needham, MA	—					18,095		66,905		14,870			19,092		80,778		—		—		99,870		20,193		2000	-1
	355 Main Street (formerly Five Cambridge Center)		Office		Cambridge, MA	—					18,863		53,346		25,899			21,173		76,935		—		—		98,108		20,864		1981/1996/2013	-1
	90 Broadway (formerly Four Cambridge Center)		Office		Cambridge, MA	—					19,104		52,078		13,680			20,785		64,077		—		—		84,862		12,426		1983/1998/2013	-1
	Waltham Weston Corporate Center		Office		Waltham, MA	—					10,385		60,694		8,105			11,097		68,087		—		—		79,184		22,151		2003	-1
	230 CityPoint		Office		Waltham, MA	—					13,189		49,823		15,744			13,593		65,163		—		—		78,756		19,029		1992	-1
	77 CityPoint		Office		Waltham, MA	—					13,847		60,383		3,163			13,873		63,520		—		—		77,393		14,639		2008	-1
	North First Business Park		Office		San Jose, CA	—					58,402		13,069		4,028			23,377		16,214		35,908		—		75,499		12,174		1981	-1
	300 Binney Street (formerly Seventeen Cambridge Center)		Office		Cambridge, MA	—					18,080		51,262		165			18,080		51,427		—		—		69,507		2,938		2013	-1
	2440 West El Camino Real		Office		Mountain View, CA	—					16,741		51,285		716			16,741		52,001		—		—		68,742		6,400		1987/2003	-1
	Wisconsin Place		Office		Chevy Chase, MD	—					—		53,349		12,787			—		66,136		—		—		66,136		14,300		2009	-1
	Reston Corporate Center		Office		Reston, VA	—					9,135		50,857		5,361			10,148		55,205		—		—		65,353		22,540		1984	-1
	New Dominion Technology Park, Bldg. Two		Office		Herndon, VA	—					5,584		51,868		3,911			6,510		54,853		—		—		61,363		18,312		2004	-1
	Sumner Square		Office		Washington, DC	—					624		28,745		25,004			1,478		52,895		—		—		54,373		25,152		1985	-1
	New Dominion Technology Park, Bldg. One		Office		Herndon, VA	40,975					3,880		43,227		3,883			4,583		46,407		—		—		50,990		21,158		2001	-1
	200 West Street		Office		Waltham, MA	—					16,148		24,983		9,782			16,813		34,100		—		—		50,913		16,439		1999	-1
	191 Spring Street		Office		Lexington, MA	—					2,850		27,166		18,558			3,151		45,423		—		—		48,574		31,087		1971/1995	-1
	255 Main Street (formerly One Cambridge Center)		Office		Cambridge, MA	—					134		25,110		20,596			548		45,292		—		—		45,840		27,168		1987	-1
	University Place		Office		Cambridge, MA	12,290					—		37,091		6,794			390		43,495		—		—		43,885		22,156		1985	-1
	2600 Tower Oaks Boulevard		Office		Rockville, MD	—					4,243		31,125		7,924			4,785		38,507		—		—		43,292		16,993		2001	-1
	Quorum Office Park		Office		Chelmsford, MA	—					3,750		32,454		5,814			5,187		36,831		—		—		42,018		13,710		2001	-1
	500 E Street		Office		Washington, DC	—					109		22,420		13,020			2,379		33,170		—		—		35,549		21,601		1987	-1
	150 Broadway (formerly Eight Cambridge Center)		Office		Cambridge, MA	—					850		25,042		8,507			1,323		33,076		—		—		34,399		12,606		1999	-1
325 Main Street (formerly Three Cambridge Center)		Office		Cambridge, MA	—					174		12,200		11,774			965		23,183		—		—		24,148		10,496		1987/2013	-1
105 Broadway (formerly Ten Cambridge Center)		Office		Cambridge, MA	—					1,299		12,943		6,851			2,395		18,698		—		—		21,093		11,948		1990	-1
40 Shattuck Road		Office		Andover, MA	—					709		14,740		2,697			893		17,253		—		—		18,146		6,171		2001	-1
201 Spring Street		Office		Lexington, MA	—					2,849		15,303		(154		)	3,124		14,874		—		—		17,998		6,511		1997	-1
Lexington Office Park		Office		Lexington, MA	—					998		1,426		13,941			1,264		15,101		—		—		16,365		11,346		1982	-1
92-100 Hayden Avenue		Office		Lexington, MA	—					594		6,748		7,819			802		14,359		—		—		15,161		10,665		1985	-1
91 Hartwell Avenue		Office		Lexington, MA	—					784		6,464		6,747			941		13,054		—		—		13,995		7,655		1985	-1
181 Spring Street		Office		Lexington, MA	—					1,066		9,520		1,950			1,160		11,376		—		—		12,536		4,306		1999	-1
195 West Street		Office		Waltham, MA	—					1,611		6,652		4,267			1,858		10,672		—		—		12,530		6,626		1990	-1
33 Hayden Avenue		Office		Lexington, MA	—					266		3,234		8,787			425		11,862		—		—		12,287		5,558		1979	-1
145 Broadway (formerly Eleven Cambridge Center)		Office		Cambridge, MA	—					121		5,535		5,278			324		10,610		—		—		10,934		7,566		1984	-1
7501 Boston Boulevard, Building Seven		Office		Springfield, VA	—					665		9,273		520			791		9,667		—		—		10,458		4,113		1997	-1
7435 Boston Boulevard, Building One		Office		Springfield, VA	—					392		3,822		3,798			659		7,353		—		—		8,012		5,794		1982	-1
7450 Boston Boulevard, Building Three		Office		Springfield, VA	—					1,165		4,681		1,880			1,430		6,296		—		—		7,726		3,056		1987	-1
8000 Grainger Court, Building Five		Office		Springfield, VA	—					366		4,282		2,684			601		6,731		—		—		7,332		5,142		1984	-1
453 Ravendale Drive		Office		Mountain View, CA	—					5,477		1,090		230			5,477		1,320		—		—		6,797		213		1977	-1
7500 Boston Boulevard, Building Six		Office		Springfield, VA	—					138		3,749		2,538			406		6,019		—		—		6,425		4,265		1985	-1
7601 Boston Boulevard, Building Eight		Office		Springfield, VA	—					200		878		4,826			551		5,353		—		—		5,904		3,688		1986	-1
250 Binney Street (formerly Fourteen Cambridge Center)		Office		Cambridge, MA	—					110		4,483		1,223			273		5,543		—		—		5,816		3,943		1983	-1
7300 Boston Boulevard, Building Thirteen		Office		Springfield, VA	—					608		4,773		230			661		4,950		—		—		5,611		4,229		2002	-1
8000 Corporate Court, Building Eleven		Office		Springfield, VA	—					136		3,071		1,611			775		4,043		—		—		4,818		2,494		1989	-1
7375 Boston Boulevard, Building Ten		Office		Springfield, VA	—					23		2,685		1,003			93		3,618		—		—		3,711		2,362		1988	-1
7374 Boston Boulevard, Building Four		Office		Springfield, VA	—					241		1,605		1,729			398		3,177		—		—		3,575		2,235		1984	-1
7451 Boston Boulevard, Building Two		Office		Springfield, VA	—					249		1,542		1,661			613		2,839		—		—		3,452		2,135		1982	-1
32 Hartwell Avenue		Office		Lexington, MA	—					168		1,943		809			314		2,606		—		—		2,920		1,685		1968-1979/1987	-1
164 Lexington Road		Office		Billerica, MA	—					592		1,370		319			643		1,638		—		—		2,281		745		1982	-1
17 Hartwell Avenue		Office		Lexington, MA	—					26		150		1,018			65		1,113		16		—		1,194		714		1968	-1
Residences on The Avenue,		Residential		Washington, DC	—					—		119,874		(28		)	—		119,846		—		—		119,846		11,039		2011	-1
2221 I St., NW
The Avant at Reston Town Center		Residential		Reston, VA	—					20,350		91,995		—			20,350		91,995		—		—		112,345		2,397		2014	-1
The Lofts at Atlantic Wharf		Residential		Boston, MA	—					3,529		54,891		1,543			3,529		56,434		—		—		59,963		5,093		2011	-1
Boston Marriott Cambridge (formerly Cambridge Center Marriott)		Hotel		Cambridge, MA	—					478		37,918		36,160			1,201		73,355		—		—		74,556		47,196		1986	-1
Cambridge Center East Garage		Garage		Cambridge, MA	—					—		35,035		1,561			103		36,493		—		—		36,596		7,647		1984	-1
Cambridge Center West Garage		Garage		Cambridge, MA	—					1,256		15,697		1,571			1,434		17,090		—		—		18,524		3,815		2006	-1
Cambridge Center North Garage		Garage		Cambridge, MA	—					1,163		11,633		2,770			1,579		13,987		—		—		15,566		8,531		1990	-1
Salesforce Tower		Development		San Francisco, CA	—					—		—		345,303			—		—		—		345,303		345,303		—		N/A	N/A
601 Massachusetts Avenue		Development		Washington, DC	—					—		—		189,173			12		—		—		189,161		189,173		—		N/A	N/A
535 Mission Street		Development		San Francisco, CA	—					—		—		171,096			10,789		39,307		—		121,000		171,096		225		N/A	N/A
10 CityPoint		Development		Waltham, MA	—					—		—		21,533			163		—		—		21,370		21,533		—		N/A	N/A
690 Folsom Street		Development		San Francisco, CA	—					—		—		13,237			1,777		6,546		—		4,914		13,237		11		N/A	N/A
Springfield Metro Center		Land		Springfield, VA	—					—		—		32,445			—		—		32,445		—		32,445		—		N/A	N/A
Reston Signature Site		Land		Reston, VA	—					—		—		30,256			—		—		30,256		—		30,256		—		N/A	N/A
Plaza at Almaden		Land		San Jose, CA	—					—		—		29,006			—		—		29,006		—		29,006		—		N/A	N/A
Tower Oaks Master Plan		Land		Rockville, MD	—					—		—		28,919			—		—		28,919		—		28,919		—		N/A	N/A
214 Third Avenue (formerly Prospect Hill)		Land		Waltham, MA	—					—		—		23,255			—		132		12,956		10,167		23,255		—		N/A	N/A
Washingtonian North		Land		Gaithersburg, MD	—					—		—		18,813			—		—		18,813		—		18,813		—		N/A	N/A
6601 & 6605 Springfield Center Drive		Land		Springfield, VA	—					—		—		13,866			—		—		13,866		—		13,866		—		N/A	N/A
103 Fourth Avenue		Land		Waltham, MA	—					—		—		11,920			—		—		11,920		—		11,920		—		N/A	N/A
Reston Gateway		Land		Reston, VA	—					—		—		9,933			—		—		9,933		—		9,933		—		N/A	N/A
Reston Eastgate		Land		Reston, VA	—					—		—		8,817			—		—		8,817		—		8,817		—		N/A	N/A
Crane Meadow		Land		Marlborough, MA	—					—		—		8,726			—		—		8,726		—		8,726		—		N/A	N/A
Broad Run Business Park		Land		Loudoun County, VA	—					—		—		6,311			—		—		6,311		—		6,311		—		N/A	N/A
20 CityPoint		Land		Waltham, MA	—					—		—		4,801			—		—		4,801		—		4,801		—		N/A	N/A
Cambridge Master Plan		Land		Cambridge, MA	—					—		—		3,527			—		—		3,527		—		3,527		—		N/A	N/A
North First Master Plan		Land		San Jose, CA	—					—		—		1,664			—		—		1,664		—		1,664		—		N/A	N/A
425 Fourth Street		Land		San Francisco, CA	—					—		—		1,261			—		—		1,261		—		1,261		—		N/A	N/A
30 Shattuck Road		Land		Andover, MA	—					—		—		1,213			—		—		1,213		—		1,213		—		N/A	N/A
					$	4,309,484		(2	)	$	4,661,817	$	11,331,324	$	3,215,276		$	4,785,772	$	13,418,220	$	268,114	$	736,311	$	19,208,417	$	3,529,978
Note: Total Real Estate does not include Furniture, Fixtures and Equipment totaling approximately $27,986. Accumulated Depreciation does not include approximately $17,681 of accumulated depreciation related to Furniture, Fixtures and Equipment.
The aggregate cost and accumulated depreciation for tax purposes was approximately $14.3 billion and $2.6 billion, respectively.
-1	Depreciation of the buildings and improvements are calculated over lives ranging from the life of the lease to 40 years.
-2	Includes the unamortized balance of the historical fair value adjustment totaling approximately $138.7 million.
A summary of activity for real estate and accumulated depreciation is as follows:
		2014			2013				2012
Real Estate:
Balance at the beginning of the year		$	18,953,601		$	14,869,887			$	13,363,113
Additions to/improvements of real estate		594,296			4,410,622				1,602,583
Assets sold/written-off		(339,480		)	(326,908		)		(95,809		)
Balance at the end of the year		$	19,208,417		$	18,953,601			$	14,869,887
Accumulated Depreciation:
Balance at the beginning of the year		$	3,145,701		$	2,919,479			$	2,626,324
Depreciation expense		456,176			419,908				367,625
Assets sold/written-off		(71,899		)	(193,686		)		(74,470		)
Balance at the end of the year		$	3,529,978		$	3,145,701			$	2,919,479
Note: Real Estate and Accumulated Depreciation amounts do not include Furniture, Fixtures and Equipment.

Organization_Details

Organization (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
	Vehicles
	acre
	sqft
	Y
Real Estate Properties [Line Items]
General and limited partnership interest in the operating partnership (percent)	89.50%	89.50%
Restriction on redemption of OP units from date of issuance (years)	1
One OP unit is equivalent to one share of Common Stock (in shares)	1
OP unit conversion rate (in shares)	1
Number Of series Of preferred units outstanding	2
Number of vehicles in structured parking	43,824
Area of parking (in square feet)	15,000,000
Area of undeveloped land parcels owned (in acres)	490.8
Preferred stock, shares outstanding	0	0
Commercial Real Estate Properties [Member]
Real Estate Properties [Line Items]
Number of real estate properties	169
Net Rentable Area	45,800,000
Total Properties Under Construction [ Member]
Real Estate Properties [Line Items]
Number of real estate properties	10
Net Rentable Area	3,300,000
Total Office Properties [ Member]
Real Estate Properties [Line Items]
Number of real estate properties	160
Class A Office Properties [Member]
Real Estate Properties [Line Items]
Number of real estate properties	129
Office Properties Under Construction [ Member]
Real Estate Properties [Line Items]
Number of real estate properties	9
Office Technical Properties [Member]
Real Estate Properties [Line Items]
Number of real estate properties	31
Hotel Property [ Member]
Real Estate Properties [Line Items]
Number of real estate properties	1
Retail Properties [ Member]
Real Estate Properties [Line Items]
Number of real estate properties	5
Retail property under construction [Member]
Real Estate Properties [Line Items]
Number of real estate properties	1
Residential Properties [Member]
Real Estate Properties [Line Items]
Number of real estate properties	3
Series Four Preferred Units [Member]
Real Estate Properties [Line Items]
Preferred Units Liquidation Preference	50
Quarterly Distributions to Series Four Preferred Units	0.25
Series Four Preferred Units Annual Dividend Payable Rate	2.00%
Series B Cumulative Redeemable Preferred Stock [Member]
Real Estate Properties [Line Items]
Preferred stock, shares outstanding	80,000	80,000
Preferred Stock, Dividend Rate, Percentage	5.25%
Depositary shares of Series B Cumulative Redeemable Preferred [Member]
Real Estate Properties [Line Items]
Preferred stock, shares outstanding	8,000,000
Series B Preferred Units [Member]
Real Estate Properties [Line Items]
Preferred stock, shares outstanding	80,000

Summary_Of_Significant_Account3

Summary Of Significant Accounting Policies (Narrative) (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
	Y
Summary Of Significant Accounting Policies [Line Items]
Net Difference Between Tax Basis And Company's Assets And Liabilities.	$2,200,000,000	$1,700,000,000
Misstatement of real estate operating expense	1,900,000
Interest and Other Income	8,765,000	8,310,000	10,091,000
Investments in and Advance to Affiliates, Subsidiaries, Associates, and Joint Ventures	193,394,000	140,097,000
Probable Sale Term of Property After Classified As Held For Sale (Years)	1
Maximum Period After Construction The Company Considers A Construction Project As Substantially Completed (In Years)	1
Interest costs capitalized	52,476,000	68,152,000	44,278,000
Salaries And Related Costs Capitalized	8,500,000	7,700,000	7,100,000
Cash FDIC Insured Limit	250,000
Separate Unrestricted Cash For Deferred Compensation Plan.	19,500,000	16,600,000
Gains from investments in securities	1,038,000	2,911,000	1,389,000
Internal Leasing Salaries And Related Costs Capitalized	6,000,000	5,100,000	5,600,000
Straight-line rent adjustments	-63,100,000	-65,800,000	-77,600,000
Above and below market rent adjustments, net	48,300,000	28,000,000	14,600,000
Ground Lease Expiration Date, Maximum	2068
REIT Annual Taxable Income Distribution Requirement Percentage, Minimum	90.00%
Company Annual Taxable Income Distribution Percentage	100.00%
Unconsolidated Joint Ventures [Member]
Summary Of Significant Accounting Policies [Line Items]
Investments in and Advance to Affiliates, Subsidiaries, Associates, and Joint Ventures	($11,400,000)	($14,000,000)

Summary_Of_Significant_Account4

Summary Of Significant Accounting Policies Schedule of Estimated Useful Lives (Details)	12 Months Ended
	Dec. 31, 2014
Land Improvements [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Estimated useful life (years)	25 years
Land Improvements [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Estimated useful life (years)	40 years
Building and Building Improvements [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Estimated useful life (years)	10 years
Building and Building Improvements [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Estimated useful life (years)	40 years
Tenant Improvement [Member]
Property, Plant and Equipment [Line Items]
Estimated useful life, description	Shorter of useful life or terms of related lease
Furniture, fixtures and equipment [Member] | Minimum [Member]
Property, Plant and Equipment [Line Items]
Estimated useful life (years)	3 years
Furniture, fixtures and equipment [Member] | Maximum [Member]
Property, Plant and Equipment [Line Items]
Estimated useful life (years)	7 years

Summary_Of_Significant_Account5

Summary Of Significant Accounting Policies (Carrying Value Of Indebtedness And Corresponding Estimate Of Fair Value) (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
Summary Of Significant Accounting Policies [Line Items]
Mortgage notes payable	$4,309,484,000	$4,449,734,000
Mezzanine notes payable	309,796,000	311,040,000
Unsecured senior notes	5,287,704,000	5,835,854,000
Unsecured exchangeable senior notes	0	744,880,000
Total	4,170,773,000
Impact of accounting pronouncement	0	2,400,000
Estimated Fair Value [Member]
Summary Of Significant Accounting Policies [Line Items]
Mortgage notes payable	4,449,541,000	4,545,283,000
Mezzanine notes payable	306,156,000	311,064,000
Unsecured senior notes	5,645,819,000	6,050,517,000
Unsecured exchangeable senior notes	0	750,266,000
Total	10,401,516,000	11,657,130,000
Carrying Amount [Member]
Summary Of Significant Accounting Policies [Line Items]
Mortgage notes payable	4,309,484,000	4,449,734,000
Mezzanine notes payable	309,796,000	311,040,000
Unsecured senior notes	5,287,704,000	5,835,854,000
Unsecured exchangeable senior notes	0	744,880,000	[1]
Total	9,906,984,000	11,341,508,000
ASC 470-20 Adjustment [Member]
Summary Of Significant Accounting Policies [Line Items]
Impact of accounting pronouncement		$2,400,000
[1]	(1)Includes the net adjustment for the equity component allocation totaling approximately $2.4 million at December 31, 2013.

Summary_Of_Significant_Account6

Summary Of Significant Accounting Policies Schedule of Tax Treatment Of Common Dividends Per Share For Federal Tax Purposes (Details) (USD $)	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012	Dec. 08, 2014
Tax Treatment Of Common Dividends Per Share For Federal Tax Purposes [Line Items]
Dividends Payable, Amount Per Share						$0.65
Special dividend (per share amount)	$4.50		$2.25
Dividends Treatment For Federal Tax Purposes, Percentage	100.00%		100.00%		100.00%
Dividends treatment for federal tax purposes.	$6.82	[1]	$4.75	[2]	$2.37
Dividend Declared [Member]
Tax Treatment Of Common Dividends Per Share For Federal Tax Purposes [Line Items]
Dividends Payable, Amount Per Share	$5.15		$2.90
non-special dividend [Member]
Tax Treatment Of Common Dividends Per Share For Federal Tax Purposes [Line Items]
Dividends Payable, Amount Per Share	$0.65		$0.65
Allocated to fiscal two thousand thirteen [Member]
Tax Treatment Of Common Dividends Per Share For Federal Tax Purposes [Line Items]
Special dividend (per share amount)	$4.41		$2.44
Capital Gains Income [Member]
Tax Treatment Of Common Dividends Per Share For Federal Tax Purposes [Line Items]
Dividends Treatment For Federal Tax Purposes, Percentage	100.00%		51.29%		3.55%
Dividends treatment for federal tax purposes.	$6.82		$2.44		$0.08
Ordinary Income [Member]
Tax Treatment Of Common Dividends Per Share For Federal Tax Purposes [Line Items]
Dividends Treatment For Federal Tax Purposes, Percentage	0.00%		48.71%		96.45%
Dividends treatment for federal tax purposes.	$0		$2.31		$2.29
Allocated to fiscal two thousand fourteen [Member]
Tax Treatment Of Common Dividends Per Share For Federal Tax Purposes [Line Items]
Special dividend (per share amount)	$0.74		$0.46
[1]	(1) The fourth quarter 2014 dividend of $5.15 per common share consists of a $4.50 per common share special dividend and a $0.65 per common share regular quarterly dividend, approximately $4.41 per common share was allocable to 2014 and approximately $0.74 per common share is allocable to 2015.
[2]	(2) The fourth quarter 2013 dividend of $2.90 per common share consists of a $2.25 per common share special dividend and a $0.65 per common share regular quarterly dividend, approximately $2.44 per common share was allocable to 2013 and approximately $0.46 per common share is allocable to 2014.

Summary_Of_Significant_Account7

Summary Of Significant Accounting Policies Schedule Of Future Contractual Minimum Lease Payments Under Non-Cancelable Ground Leases (Details) (USD $)	Dec. 31, 2014
In Thousands, unless otherwise specified
Summary of Significant Accounting Policies [Abstract]
2015	$13,507
2016	13,732
2017	13,963
2018	14,198
2019	14,461
Thereafter	$884,726

Summary_Of_Significant_Account8

Summary Of Significant Accounting Policies GAAP Reconciliation Of Net Income To Taxable Income (Details) (USD $)	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013	Dec. 31, 2012
Reconciliation of GAAP net income to taxable income [Abstract]
One-time deduction for Tangible Repair Regulations	$385,600,000
Net income attributable to Boston Properties, Inc.	443,611,000		749,811,000	289,650,000
Straight-line rent adjustments	-91,733,000		-74,445,000	-80,281,000
Book/Tax differences from depreciation and amortization	239,681,000		170,370,000	105,599,000
Income Tax Reconciliation, Book/Tax Differences On Interest Expense	-43,148,000		-7,912,000	27,660,000
Book/Tax differences on gains/losses from capital transactions	943,778,000		-124,413,000	-22,408,000
Book/Tax differences from stock-based compensation	32,483,000		42,146,000	19,660,000
Income Tax Reconciliation, Book/Tax Differences On Tngible Property Regulations	-442,650,000	[1]	0	0
Other book/tax differences, net	-7,945,000		-4,885,000	7,801,000
Taxable income	$1,074,077,000		$750,672,000	$347,681,000
[1]	(1) In September 2013, the Internal Revenue Service released final Regulations governing when taxpayers like the Company must capitalize and depreciate costs for acquiring, maintaining, repairing and replacing tangible property and when taxpayers can deduct such costs. These final Regulations are effective for tax years beginning on or after January 1, 2014. These Regulations permitted the Company to deduct certain types of expenditures that were previously required to be capitalized. The Regulations also allowed the Company to make a one-time election to immediately deduct certain amounts that were capitalized in previous years that are not required to be capitalized under the new Regulations.

Summary_Of_Significant_Account9

Summary Of Significant Accounting Policies Schedule of Finite Lived Intangible Assets Future Amortization Expense (Details) (USD $)	Dec. 31, 2014
In Thousands, unless otherwise specified
Finite-Lived Intangible Assets [Line Items]
2015	$66,390
2016	55,327
2017	38,812
2018	33,964
2019	27,439
Acquired Above-Market Lease Intangibles [Member]
Finite-Lived Intangible Assets [Line Items]
2015	22,671
2016	20,491
2017	12,277
2018	8,637
2019	7,106
Acquired Below-Market Lease Intangibles [Member]
Finite-Lived Intangible Assets [Line Items]
2015	57,019
2016	51,460
2017	35,896
2018	33,215
2019	$27,615

Real_Estate_Narrative_Details

Real Estate (Narrative) (Details) (USD $)

3 Months Ended

12 Months Ended

0 Months Ended

9 Months Ended

0 Months Ended

7 Months Ended

0 Months Ended

9 Months Ended

0 Months Ended

Dec. 31, 2014

Sep. 30, 2014

Jun. 30, 2014

Mar. 31, 2014

Dec. 31, 2013

Sep. 30, 2013

Jun. 30, 2013

Mar. 31, 2013

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2012

Apr. 10, 2013

Dec. 31, 2013

31-May-13

Dec. 31, 2013

Jul. 29, 2014

Aug. 20, 2014

Aug. 22, 2014

Oct. 02, 2014

Oct. 03, 2014

Oct. 24, 2014

Dec. 30, 2014

23-May-11

Apr. 10, 2014

Oct. 30, 2014

Nov. 06, 2014

Nov. 12, 2014

Oct. 22, 2014

Aug. 31, 2014

Sep. 17, 2014

20-May-14

Apr. 01, 2014

Feb. 10, 2014

Nov. 01, 2014

Apr. 03, 2014

Oct. 09, 2013

Feb. 07, 2014

Feb. 26, 2013

Buildings

partners

Y

floor

sqft

sqft

sqft

sqft

sqft

sqft

sqft

sqft

sqft

sqft

sqft

sqft

sqft

sqft

apartments

Real Estate Properties [Line Items]

Gains on consolidation of joint ventures

$0

$385,991,000

$0

Payments to Acquire and Develop Real Estate

405,942,000

396,835,000

356,397,000

Revenues contributed by acquisitions

613,707,000

618,803,000

589,794,000

574,694,000

576,199,000

571,481,000

510,033,000

477,826,000

2,396,998,000

2,135,539,000

1,847,186,000

Earnings

801,822,000

678,120,000

640,321,000

Gain on forgiveness of debt from discontinued operations

0

20,182,000

0

Proceeds from sales of real estate

419,864,000

250,078,000

61,963,000

Gains on sales of real estate

168,039,000

0

0

Gains on sales of real estate from discontinued operations

0

112,829,000

36,877,000

Impairment loss from discontinued operations

0

3,241,000

0

Impairment loss

0

8,306,000

0

Development and management services

25,316,000

29,695,000

34,060,000

Mountain View Research And Technology Parks [Member]

Real Estate Properties [Line Items]

Gains on consolidation of joint ventures

26,500,000

Revenues contributed by acquisitions

16,700,000

Earnings

400,000

Number of real estate properties

16

Net Rentable Area

604,000

Aggregate purchase price

233,084,000

767 5th Avenue (The General Motors Building) [Member]

Real Estate Properties [Line Items]

Gains on consolidation of joint ventures

359,500,000

Revenues contributed by acquisitions

168,400,000

Earnings

8,400,000

Net Rentable Area

1,800,000

Number of Joint Venture Partners

2

767 5th Avenue (The General Motors Building) [Member] | Consolidated Properties [Member]

Real Estate Properties [Line Items]

Partners Joint Venture Ownership Percentage

40.00%

Four Two Five Fourth Street [Member]

Real Estate Properties [Line Items]

Non-refundable option payment

1,000,000

804 carnegie center [Member]

Real Estate Properties [Line Items]

Aggregate purchase price

3,700,000

Area of Real Estate Property

130,000

Mountain View Technology Park and Mountain View Research Building Sixteen [Member]

Real Estate Properties [Line Items]

Proceeds from sales of real estate

90,600,000

Gains on sales of real estate

35,900,000

Sale Price Of Sold Property

92,100,000

Land at One Reston Overlook [Member]

Real Estate Properties [Line Items]

Proceeds from sales of real estate

2,600,000

Gains on sales of real estate

1,200,000

Land Parcel Within Broad Run Business Park [Member]

Real Estate Properties [Line Items]

Proceeds from sales of real estate

9,700,000

Gains on sales of real estate

4,300,000

Area of Land

15.5

Sale Price Of Sold Property

9,800,000

Patriots park [Member]

Real Estate Properties [Line Items]

Earnings

10,800,000

5,300,000

8,200,000

Proceeds from sales of real estate

319,100,000

Gains on sales of real estate

84,600,000

Rental revenue supoprt payments to buyer maximum

12,300,000

Number of real estate properties

3

Sale Price Of Sold Property

321,000,000

Area of Real Estate Property

706,000

415 Main Street (formerly Seven Cambridge Center) [Member]

Real Estate Properties [Line Items]

Area of Real Estate Property

231,000

130 Third Avenue [Member]

Real Estate Properties [Line Items]

Proceeds from sales of real estate

13,600,000

Gains on sales of real estate

8,300,000

Sale Price Of Sold Property

14,300,000

Area of Real Estate Property

129,000

75 Ames Street [Member]

Real Estate Properties [Line Items]

Ground lease term (Years)

99

Gains on sales of real estate

33,800,000

Area of Real Estate Property

250,000

Proceeds from the sale of a condominium interest in real estate

56,800,000

Development and management services

11,400,000

Ground lease revenue

459,000

Mountain View Research Building Sixteen [Member] | Mountain View Technology Park and Mountain View Research Building Sixteen [Member]

Real Estate Properties [Line Items]

Area of Real Estate Property

63,000

Mountain View Technology [Member] | Mountain View Technology Park and Mountain View Research Building Sixteen [Member]

Real Estate Properties [Line Items]

Number of real estate properties

7

Area of Real Estate Property

135,000

250 West 55th Street [Member]

Real Estate Properties [Line Items]

Area of Real Estate Property

988,000

Six Eighty Folsom Street [Member]

Real Estate Properties [Line Items]

Area of Real Estate Property

525,000

888 Boylston Street [Member]

Real Estate Properties [Line Items]

Area of Real Estate Property

425,000

10 CityPoint [Member]

Real Estate Properties [Line Items]

Area of Real Estate Property

245,000

99Third Avenue [Member]

Real Estate Properties [Line Items]

Area of Real Estate Property

17,000

The Avant at Reston Town Center Residential [Member]

Real Estate Properties [Line Items]

Number of apartment units

359

Area of Real Estate Property

355,000

Salesforce Tower [Member] | Consolidated Properties [Member]

Real Estate Properties [Line Items]

Number of floors in building

61

Area of Real Estate Property

1,400,000

Ownership Percentage by Parent

95.00%

Square Footage Of Signed Lease

714,000

535 Mission Street [Member]

Real Estate Properties [Line Items]

Area of Real Estate Property

307,000

415 Main Street (formerly Seven Cambridge Center) [Member]

Real Estate Properties [Line Items]

Purchase option date (lease year)

11

Purchase Option Price for Real Estate Property

106,000,000

690 Folsom Street [Member]

Real Estate Properties [Line Items]

Area of Real Estate Property

25,000

767 5th Avenue (The General Motors Building) [Member]

Real Estate Properties [Line Items]

Number of Joint Venture Partners

2

related to rental support payment [Member] | Patriots park [Member]

Real Estate Properties [Line Items]

Area of Real Estate Property

17,762

Noncontrolling Interests [Member] | Consolidated Properties [Member]

Real Estate Properties [Line Items]

Net Rentable Area

1,246,000

Number of Joint Venture Partners

2

Noncontrolling Interests [Member] | Time Square Tower [Member] | Consolidated Properties [Member]

Real Estate Properties [Line Items]

Portion allocation to noncontrolling interest based on carrying value of the total equity of the property prior to the sale

243,500,000

ownership interest sold (percent)

45.00%

Net Contractual Sales Price

684,000,000

Proceeds from sale of an interest in a real estate asset

673,100,000

Difference between net cash proceeds received and noncontrolling interest recognized

429,600,000

Noncontrolling Interests [Member] | 601 Lexington Avenue, Atlantic Wharf Office Building and 100 Federal Street [Member] | Consolidated Properties [Member]

Real Estate Properties [Line Items]

Disposal Group, Including Discontinued Operation, Secured Debt

712,900,000

Ownership Percentage

45.00%

Sale Price Of Sold Property

1,827,000,000

Proceeds from sale of an interest in a real estate asset

1,497,000,000

Difference between net cash proceeds received and noncontrolling interest recognized

648,400,000

Noncontrolling Interest, Increase from Sale of Parent Equity Interest

$849,000,000

Noncontrolling Interests [Member] | Salesforce Tower [Member] | Consolidated Properties [Member]

Real Estate Properties [Line Items]

Ownership Percentage

50.00%

5.00%

Ownership Percentage by Parent

95.00%

Noncontrolling Interests [Member] | 601 Lexington Avenue [Member] | 601 Lexington Avenue, Atlantic Wharf Office Building and 100 Federal Street [Member] | Consolidated Properties [Member]

Real Estate Properties [Line Items]

Area of Real Estate Property

1,669,000

Noncontrolling Interests [Member] | Atlantic Wharf Office [Member] | 601 Lexington Avenue, Atlantic Wharf Office Building and 100 Federal Street [Member] | Consolidated Properties [Member]

Real Estate Properties [Line Items]

Area of Real Estate Property

791,000

Noncontrolling Interests [Member] | One Hundred Federal Street [Member] | 601 Lexington Avenue, Atlantic Wharf Office Building and 100 Federal Street [Member] | Consolidated Properties [Member]

Real Estate Properties [Line Items]

Area of Real Estate Property

1,323,000

Noncontrolling Interests [Member] | Low Rise [Member] | 601 Lexington Avenue [Member] | 601 Lexington Avenue, Atlantic Wharf Office Building and 100 Federal Street [Member] | Consolidated Properties [Member]

Real Estate Properties [Line Items]

Number of floors in building

6

Noncontrolling Interests [Member] | Office Tower [Member] | 601 Lexington Avenue [Member] | 601 Lexington Avenue, Atlantic Wharf Office Building and 100 Federal Street [Member] | Consolidated Properties [Member]

Real Estate Properties [Line Items]

Number of floors in building

59

Real_Estate_Schedule_Of_Alloca

Real Estate (Schedule Of Allocation Of The Aggregate Purchase Price Of Acquisition) (Details) (USD $)	12 Months Ended			0 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Apr. 10, 2013	31-May-13
Restructuring Cost and Reserve [Line Items]
Real estate and related intangibles recorded upon consolidation	$0	$3,356,000	$0
Mortgage notes payable	-4,309,484	-4,449,734
Mezzanine notes payable	-309,796	-311,040
Outside members' notes payable	-180,000	-180,000
Debt recorded upon consolidation	0	-2,056,000	0
Cash and cash equivalents	0	79,468	0
Accrued interest payable	-163,532	-167,523
Working capital recorded upon consolidation	0	-177,315	0
Noncontrolling interest in property partnerships recorded upon consolidation	0	-480,861	0
Mountain View Research And Technology Parks [Member]
Restructuring Cost and Reserve [Line Items]
Land				126,521
Building and improvements				82,451
Tenant Improvements				7,326
In-place lease Intangibles				23,279
Above market rents				843
Below market rents				-7,336
Aggregate purchase price				233,084
767 5th Avenue (The General Motors Building) [Member]
Restructuring Cost and Reserve [Line Items]
Land					1,796,252
Building and improvements					1,447,446
Tenant Improvements					85,208
In-place lease Intangibles					357,781
Above market rents					101,897
Below market rents					-239,641
Above Market Assumed Debt Adjustments					-192,943
Real estate and related intangibles recorded upon consolidation					3,356,000
Mortgage notes payable					-1,300,000
Mezzanine notes payable					-306,000
Outside members' notes payable					-450,000	[1]
Debt recorded upon consolidation					-2,056,000
Cash and cash equivalents					79,468
Cash held in escrows					2,403
Tenant and other receivables					7,104
Prepaid expenses and other assets					4,269
Accounts payable and other expenses					-2,418
Accrued interest payable					-182,369	[2]
Other liabilities					6,304
Working capital recorded upon consolidation					-97,847
Noncontrolling Interest					-520,000
Noncontrolling Interest working capital					39,139
Noncontrolling interest in property partnerships recorded upon consolidation					-480,861
Net Assets Recorded Upon Consolidation					721,292
Consolidation, Eliminations [Member] | 767 5th Avenue (The General Motors Building) [Member]
Restructuring Cost and Reserve [Line Items]
Outside members' notes payable					-270,000
Accrued interest payable					($105,500)
[1]	(1)The Company's member loan totaling $270.0 million eliminates in consolidation.
[2]	(2)The Company's share of the accrued interest payable on the members' loans totaling approximately $105.5 million eliminates in consolidation.

Real_Estate_Pro_Forma_In_Conne

Real Estate (Pro Forma In Connection With Acquisition) (Details) (USD $)	12 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Real Estate [Abstract]
Total revenue	$2,257,098	$2,149,391
Income from continuing operations	302,354	634,457
Net income attributable to Boston Properties, Inc.	$400,017	$626,174
Basic earnings per share: Net income per share attributable to Boston Properties, Inc. (in dollars per share)	$2.58	$4.15
Diluted earnings per share: Net income per share attributable to Boston Properties, Inc. (in dollars per share)	$2.57	$4.13

Real_Estate_Summary_Of_The_Dis

Real Estate (Summary Of The Discontinued Operations (Details) (USD $)	3 Months Ended								12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total revenue	$613,707	$618,803	$589,794	$574,694	$576,199	$571,481	$510,033	$477,826	$2,396,998	$2,135,539	$1,847,186
Expenses
Depreciation and amortization									628,573	560,637	445,875
Total expenses									1,595,176	1,457,419	1,206,865
Operating income									801,822	678,120	640,321
Other Expenses
Interest expense									455,743	446,880	410,970
Noncontrolling interest in income from discontinued operations – common units of the Operating Partnership									0	-14,151	-5,075
Income from discontinued operations attributable to Boston Properties, Inc.										123,641	41,608
Gains on sales of real estate from discontinued operations									0	112,829	36,877
Gain on forgiveness of debt from discontinued operations									0	20,182	0
Impairment loss from discontinued operations									0	-3,241	0
10 & 20 Burlington Mall Road, One Preserve Parkway, 1301 New York Avenue, 303 Almaden Blvd, Montvale Center and Bedford Business Park [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total revenue											32,607
Expenses
Operating											12,038
Depreciation and amortization											8,169
Total expenses											20,207
Operating income											12,400
Other Expenses
Interest expense											2,594
Income from discontinued operations											9,806
Noncontrolling interest in income from discontinued operations – common units of the Operating Partnership											-1,031
Income from discontinued operations attributable to Boston Properties, Inc.											8,775
Gains on sales of real estate from discontinued operations											36,877
Gain on forgiveness of debt from discontinued operations											0
Impairment loss from discontinued operations											0
Noncontrolling interest in gains on sales of real estate, gain on forgiveness of debt and impairment loss from discontinued operations – common units of the Operating Partnership											-4,044
Gain on the Slae of Real Estate, Gain On Forgiveness of Debt and Impairment Losses from Discontinued Operations Attributable to Company											32,833
Montvale Center, 303 Almaden Blvd, 1301 New York Avenue, One Preserve Parkway and 10 & 20 Burlington Mall Road [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total revenue										20,138
Expenses
Operating										6,996
Depreciation and amortization										4,760
Total expenses										11,756
Operating income										8,382
Other Expenses
Interest expense										360
Income from discontinued operations										8,022
Noncontrolling interest in income from discontinued operations – common units of the Operating Partnership										-803
Income from discontinued operations attributable to Boston Properties, Inc.										7,219
Gains on sales of real estate from discontinued operations										112,829
Gain on forgiveness of debt from discontinued operations										20,182
Impairment loss from discontinued operations										-3,241
Noncontrolling interest in gains on sales of real estate, gain on forgiveness of debt and impairment loss from discontinued operations – common units of the Operating Partnership										-13,348
Gain on the Slae of Real Estate, Gain On Forgiveness of Debt and Impairment Losses from Discontinued Operations Attributable to Company										$116,422

Real_Estate_Schedule_of_Real_E

Real Estate Schedule of Real Estate Properties (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Real Estate [Abstract]
Land	$4,785,772	$4,450,532
Land held for future development	268,114	297,376
Buildings and improvements	11,666,105	11,065,113
Tenant improvements	1,752,115	1,617,401
Furniture, fixtures and equipment	27,986	25,164
Construction in progress	736,311	1,523,179
Total	19,236,403	18,978,765
Less: accumulated depreciation	-3,547,659	-3,161,571
Total	$15,688,744	$15,817,194

Deferred_Charges_Details

Deferred Charges (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Deferred Charges [Abstract]
Leasing costs, including lease related intangibles	$1,234,192	$1,183,204
Financing costs	69,127	76,798
Deferred charges, gross	1,303,319	1,260,002
Less: Accumulated amortization	-471,575	-375,552
Deferred charges, total	$831,744	$884,450

Deferred_Charges_Schedule_of_A

Deferred Charges Schedule of Acquired In-Place Lease Intangibles (Details) (USD $)	Dec. 31, 2014
In Thousands, unless otherwise specified
Deferred Charges [Abstract]
2015	$66,390
2016	55,327
2017	38,812
2018	33,964
2019	$27,439

Investments_in_Unconsolidated_2

Investments in Unconsolidated Joint Ventures (Investments in Unconsolidated Joint Ventures) (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013	Apr. 10, 2014
				sqft
Schedule of Equity Method Investments [Line Items]
Investments in and Advance to Affiliates, Subsidiaries, Associates, and Joint Ventures	$193,394		$140,097
Investments In Affiliates Subsidiaries Associates And Joint Ventures, net	182,042	[1]
Unconsolidated Joint Ventures [Member]
Schedule of Equity Method Investments [Line Items]
Investments in and Advance to Affiliates, Subsidiaries, Associates, and Joint Ventures	-11,400		-14,000
Square 407 Limited Partnership
Schedule of Equity Method Investments [Line Items]
Properties	Market Square North
Equity Method Investment, Ownership Percentage	50.00%
Investments in and Advance to Affiliates, Subsidiaries, Associates, and Joint Ventures	-8,022	[1]
The Metropolitan Square Associates LLC
Schedule of Equity Method Investments [Line Items]
Properties	Metropolitan Square
Equity Method Investment, Ownership Percentage	51.00%
Investments in and Advance to Affiliates, Subsidiaries, Associates, and Joint Ventures	8,539
BP/CRF 901 New York Avenue LLC
Schedule of Equity Method Investments [Line Items]
Properties	901 New York Avenue
Equity Method Investment, Ownership Percentage	25.00%	[2]
Investments in and Advance to Affiliates, Subsidiaries, Associates, and Joint Ventures	-1,080	[1]
WP Project Developer LLC
Schedule of Equity Method Investments [Line Items]
Properties	Wisconsin Place Land and Infrastructure
Equity Method Investment, Ownership Percentage	33.30%	[3]
Investments in and Advance to Affiliates, Subsidiaries, Associates, and Joint Ventures	45,514
Annapolis Junction NFM, LLC
Schedule of Equity Method Investments [Line Items]
Properties	Annapolis Junction
Equity Method Investment, Ownership Percentage	50.00%	[4]
Investments in and Advance to Affiliates, Subsidiaries, Associates, and Joint Ventures	25,246
Number of real estate properties	2
Number of buildings under construction	2
Parcels of undeveloped land	2
540 Madison Venture LLC
Schedule of Equity Method Investments [Line Items]
Properties	540 Madison Avenue
Equity Method Investment, Ownership Percentage	60.00%
Investments in and Advance to Affiliates, Subsidiaries, Associates, and Joint Ventures	68,128
500 North Capitol LLC
Schedule of Equity Method Investments [Line Items]
Properties	500 North Capitol Street, NW
Equity Method Investment, Ownership Percentage	30.00%
Investments in and Advance to Affiliates, Subsidiaries, Associates, and Joint Ventures	-2,250	[1]
1001 6th Street (formerly five zero one K Street ) [Member]
Schedule of Equity Method Investments [Line Items]
Properties	1001 6th Street formerly 501 K Street)
Equity Method Investment, Ownership Percentage	50.00%	[5]
Investments in and Advance to Affiliates, Subsidiaries, Associates, and Joint Ventures	41,736
Potential additonal payments to joint venture partner	2
Minimum square footage to make a potential additional payment to joint venture partner (in sqft)	520,000			520,000
Entity Owning Land And Infrastructure Of Project [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	33.30%
Podium Developer LLC [Member]
Schedule of Equity Method Investments [Line Items]
Properties	North Station (Phase I - Air Rights)
Equity Method Investment, Ownership Percentage	50.00%
Investments in and Advance to Affiliates, Subsidiaries, Associates, and Joint Ventures	$4,231
[1]	Investments with deficit balances aggregating approximately $11.4 million have been reflected within Other Liabilities on the Company's Consolidated Balance Sheets.
[2]	The Company’s economic ownership has increased based on the achievement of certain return thresholds.
[3]	The Company’s wholly-owned entity that owns the office component of the project also owns a 33.3% interest in the entity owning the land, parking garage and infrastructure of the project.
[4]	The joint venture owns two in-service buildings, two buildings under construction and two undeveloped land parcels
[5]	Under the joint venture agreement, the partner will be entitled to up to two additional payments from the venture based on increases in total square footage of the project above 520,000 square feet and achieving certain project returns at stabilization.

Investments_in_Unconsolidated_3

Investments in Unconsolidated Joint Ventures (Balance Sheets of the Unconsolidated Joint Ventures) (Details) (USD $)	Dec. 31, 2014		Dec. 31, 2013
In Thousands, unless otherwise specified
ASSETS
Real estate and development in process, net	$15,688,744		$15,817,194
LIABILITIES AND MEMBERS'/PARTNERS' EQUITY
Mortgage and notes payable	4,309,484		4,449,734
Other liabilities	502,255		592,982
Total liabilities and equity	19,886,767		20,176,264
Carrying value of the Company’s investments in unconsolidated joint ventures (2)	193,394		140,097
Unconsolidated Joint Ventures [Member]
ASSETS
Real estate and development in process, net	1,034,552		924,297
Other assets	264,097		163,149
Total assets	1,298,649		1,087,446
LIABILITIES AND MEMBERS'/PARTNERS' EQUITY
Mortgage and notes payable	830,075		749,732
Other liabilities	34,211		28,830
Members’/Partners’ equity	434,363		308,884
Total liabilities and equity	1,298,649		1,087,446
Company’s share of equity	209,828		154,726
Basis differentials (1)	-27,786	[1]	-28,642	[1]
Carrying value of the Company’s investments in unconsolidated joint ventures (2)	182,042	[2]	126,084	[2]
Unconsolidated Joint Ventures [Member]
LIABILITIES AND MEMBERS'/PARTNERS' EQUITY
Carrying value of the Company’s investments in unconsolidated joint ventures (2)	($11,400)		($14,000)
[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.
[2]	Investments with deficit balances aggregating approximately $11.4 million and $14.0 million at December 31, 2014 and 2013, respectively, have been reflected within Other Liabilities on the Company's Consolidated Balance Sheets.

Investments_in_Unconsolidated_4

Investments in Unconsolidated Joint Ventures (Statements of Operations of the Joint Ventures) (Details) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Schedule of Equity Method Investments [Line Items]
Total revenue	$613,707,000	$618,803,000	$589,794,000	$574,694,000	$576,199,000	$571,481,000	$510,033,000	$477,826,000	$2,396,998,000		$2,135,539,000		$1,847,186,000
Expenses
Depreciation and amortization									628,573,000		560,637,000		445,875,000
Total expenses									1,595,176,000		1,457,419,000		1,206,865,000
Operating income									801,822,000		678,120,000		640,321,000
Other income (expense)
Interest expense									-455,743,000		-446,880,000		-410,970,000
Losses from early extinguishments of debt									-10,633,000		122,000		-4,453,000
Income from continuing operations	85,323,000	109,038,000	95,901,000	67,756,000	79,395,000	82,311,000	503,446,000	38,496,000	358,018,000		703,648,000		285,456,000
Net income									526,057,000		841,440,000		332,139,000
Income from unconsolidated joint ventures									12,769,000		75,074,000		49,078,000
Gains on consolidation of joint ventures									0		385,991,000		0
Straight-line rent adjustments									63,100,000		65,800,000		77,600,000
Above and below market rent adjustments, net									48,300,000		28,000,000		14,600,000
Unconsolidated Joint Ventures [Member]
Schedule of Equity Method Investments [Line Items]
Total revenue									158,161,000	[1]	311,548,000	[1]	564,205,000	[1]
Expenses
Operating									62,974,000		105,319,000		162,665,000
Depreciation and amortization									37,041,000		86,088,000		163,134,000
Total expenses									100,015,000		191,407,000		325,799,000
Operating income									58,146,000		120,141,000		238,406,000
Other income (expense)
Interest expense									-31,896,000		-112,535,000		-224,645,000
Losses from early extinguishments of debt									0		-1,677,000		0
Income from continuing operations									26,250,000		5,929,000		13,761,000
Gains on sales of real estate									0		14,207,000		990,000
Net income									26,250,000		20,136,000		14,751,000
Company’s share of net income									11,913,000		4,612,000		6,863,000
Gains on sales of real estate									0		54,501,000		0
Basis differential									856,000		-1,017,000		1,732,000
Elimination of inter-entity interest on partner loan									0		16,978,000		40,483,000
Income from unconsolidated joint ventures									12,769,000		75,074,000		49,078,000
Gains on consolidation of joint ventures									0		385,991,000		0
Straight-line rent adjustments									3,000,000		7,800,000		12,000,000
Above and below market rent adjustments, net									-100,000		33,700,000		91,100,000
Lease termination income													19,600,000
Company's share of lease termination income													$11,800,000
[1]	Includes straight-line rent adjustments of $3.0 million, $7.8 million and $12.0 million for the years ended December 31, 2014, 2013 and 2012, respectively. Includes net above-/below-market rent adjustments of $(0.1) million, $33.7 million and $91.1 million for the years ended December 31, 2014, 2013 and 2012, respectively. Total revenue for the year ended December 31, 2012 includes termination income totaling approximately $19.6 million (of which the Company's share is approximately $11.8 million) related to a lease termination with a tenant at 767 Fifth Avenue (The General Motors Building).

Investment_in_unconsolidated_J

Investment in unconsolidated Joint Ventures (Narrative) (Details) (USD $)

12 Months Ended

0 Months Ended

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2012

Apr. 10, 2014

31-May-13

Jun. 23, 2014

Apr. 30, 2014

Dec. 19, 2014

Dec. 17, 2014

Oct. 24, 2014

Oct. 23, 2014

sqft

partners

Y

sqft

Schedule of Equity Method Investments [Line Items]

Payments to Acquire Equity Method Investments

$52,052,000

$0

$6,214,000

(Losses) gains from early extinguishments of debt

-10,633,000

122,000

-4,453,000

Repayments of Secured Debt

87,758,000

80,311,000

253,877,000

Gains on consolidation of joint ventures

0

385,991,000

0

1001 6th Street (formerly five zero one K Street ) [Member]

Schedule of Equity Method Investments [Line Items]

Equity Method Investment, Ownership Percentage

50.00%

[1]

Payments to Acquire Equity Method Investments

39,000,000

Net Rentable Area

520,000

Potential additonal payments to joint venture partner

2

Minimum square footage to make a potential additional payment to joint venture partner (in sqft)

520,000

520,000

767 5th Avenue (The General Motors Building) [Member]

Schedule of Equity Method Investments [Line Items]

Number of Joint Venture Partners

2

Annapolis Junction Building Six [Member]

Schedule of Equity Method Investments [Line Items]

Payments to Acquire Equity Method Investments

5,400,000

Net Rentable Area

125,000

Construction Loan Facility Borrowing Capacity

26,000,000

Debt Instrument, Basis Spread on Variable Rate

1.50%

Maturity date

23-Jun-17

Number of extensions

2

Extension Option (in years)

1

Annapolis Junction Building Six [Member]

Schedule of Equity Method Investments [Line Items]

Area of Real Estate Property

119,000

Nine zero one New York Avenue [Member]

Schedule of Equity Method Investments [Line Items]

Equity Method Investment, Ownership Percentage

25.00%

First phase of North Station [Member]

Schedule of Equity Method Investments [Line Items]

Net Rentable Area

377,000

joint venture partner [Member] | 1001 6th Street (formerly five zero one K Street ) [Member]

Schedule of Equity Method Investments [Line Items]

Equity Method Investment, Ownership Percentage

50.00%

joint venture partner [Member] | First phase of North Station [Member]

Schedule of Equity Method Investments [Line Items]

Equity Method Investment, Ownership Percentage

50.00%

Payments to Acquire Equity Method Investments

13,000,000

Company [Member] | 1001 6th Street (formerly five zero one K Street ) [Member]

Schedule of Equity Method Investments [Line Items]

Equity Method Investment, Ownership Percentage

50.00%

Company [Member] | Annapolis Junction Building Six [Member]

Schedule of Equity Method Investments [Line Items]

Equity Method Investment, Ownership Percentage

50.00%

Company [Member] | First phase of North Station [Member]

Schedule of Equity Method Investments [Line Items]

Equity Method Investment, Ownership Percentage

50.00%

Payments to Acquire Equity Method Investments

4,200,000

First Mortgage [Member] | Nine zero one New York Avenue [Member]

Schedule of Equity Method Investments [Line Items]

Coupon/Stated Rate

3.61%

Proceeds from Issuance of Secured Debt

225,000,000

Prior to refinance first mortgage loan [Member] | Nine zero one New York Avenue [Member]

Schedule of Equity Method Investments [Line Items]

Coupon/Stated Rate

5.19%

Repayments of Secured Debt

150,400,000

Construction Loans [Member] | Annapolis Junction Building Six [Member]

Schedule of Equity Method Investments [Line Items]

Debt Instrument, Description of Variable Rate Basis

LIBOR

LIBOR

Equity Method Investment, Ownership Percentage

50.00%

Construction Loan Facility Borrowing Capacity

16,400,000

Debt Instrument, Basis Spread on Variable Rate

2.25%

1.65%

Construction Loan

13,900,000

Maximum [Member] | Company [Member] | First phase of North Station [Member]

Schedule of Equity Method Investments [Line Items]

Payments to Acquire Equity Method Investments

$8,800,000

[1]

Under the joint venture agreement, the partner will be entitled to up to two additional payments from the venture based on increases in total square footage of the project above 520,000 square feet and achieving certain project returns at stabilization.

Mortgage_Notes_Payable_Schedul1

Mortgage Notes Payable Schedule Of Aggregate Principal Payments On Mortgage Notes (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Thousands, unless otherwise specified
Mortgage Notes Payable [Abstract]
2015	$26,184
2016	608,879
2017	2,821,750
2018	18,633
2019	19,670
Thereafter	675,657
Total	4,170,773
Unamortized balance of historical fair value adjustments	138,711
Total carrying value of mortgage notes payable	$4,309,484	$4,449,734

Mortgage_Notes_Payable_Narrati

Mortgage Notes Payable (Narrative) (Details) (USD $)	12 Months Ended			3 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Sep. 30, 2014	Jul. 01, 2014
Mortgage Notes Payable [Line Items]
Mortgage notes payable	$4,309,484,000	$4,449,734,000
Final maturity date of mortgage notes	10-Apr-22
Gain on forgiveness of debt from discontinued operations	0	20,182,000	0
Repayments of mortgage notes payable	87,758,000	80,311,000	253,877,000
(Losses) gains from early extinguishments of debt	-10,633,000	122,000	-4,453,000
Number Of Mortgage Loans Accounted For Fair Value	4	4
Outstanding balance of mortgage loans accounted for at fair value	2,200,000,000	2,184,800,000
Interest Expense Decrease Due To Recording Mortgage Loans At Fair Value	52,500,000	34,400,000	7,000,000
Cumulative Liability Related to the Fair Value of Debt Adjustment	138,700,000	191,200,000
Outside members' notes payable	180,000,000	180,000,000
Interest Payable	163,532,000	167,523,000
Minimum [Member]
Mortgage Notes Payable [Line Items]
Number Real Estate Assets Used For Mortgage Loan Collateral	1	1
Fixed Rate Mortgages [Member]
Mortgage Notes Payable [Line Items]
Mortgage notes payable	4,300,000,000	4,400,000,000
Contractual interest rate range, minimum	4.75%	4.75%
Contractual interest rate range, maximum	7.69%	9.93%
Contractual interest rate, weighted vverage interest rate	5.70%	5.70%
Variable Rate Mortgages [Member]
Mortgage Notes Payable [Line Items]
Number of variable rate mortgage loans	0	0
London Interbank Offered Rate	0.17%	0.17%
Mortgages [Member] | New Dominion Technology Park, Bldg. Two [Member]
Mortgage Notes Payable [Line Items]
Repayments of mortgage notes payable				63,000,000
Interest at a fixed rate					5.55%
Penalty for early repayment of debt				$0

Unsecured_Senior_Notes_Schedul

Unsecured Senior Notes Schedule of Unsecured Senior Notes (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013
Debt Instrument [Line Items]
Debt instrument principal due prior to maturity	$0
Net unamortized discount	-12,296		-14,146
Total	5,287,704		5,835,854
Senior Notes [Member]
Debt Instrument [Line Items]
Principal Amount	5,300,000
Net unamortized discount	-12,296
Total	5,287,704
Senior Notes [Member] | Ten Year Unsecured Senior Notes Five Point Nine Six Seven Percent
Debt Instrument [Line Items]
Coupon/Stated Rate	5.88%
Effective Rate	5.97%	[1]
Principal Amount	700,000
Maturity date	15-Oct-19	[2]
Senior Notes [Member] | Ten Year Unsecured Senior Notes Five Point Seven Zero Eight Percent
Debt Instrument [Line Items]
Coupon/Stated Rate	5.63%
Effective Rate	5.71%	[1]
Principal Amount	700,000
Maturity date	15-Nov-20	[2]
Senior Notes [Member] | Ten Year Unsecured Senior Notes Four Point Two Eight Nine Percent
Debt Instrument [Line Items]
Coupon/Stated Rate	4.13%
Effective Rate	4.29%	[1]
Principal Amount	850,000
Maturity date	15-May-21	[2]
Senior Notes [Member] | Seven Year Unsecured Senior Notes Three Point Eight Five Three Percent
Debt Instrument [Line Items]
Coupon/Stated Rate	3.70%
Effective Rate	3.85%	[1]
Principal Amount	850,000
Maturity date	15-Nov-18	[2]
Senior Notes [Member] | Eleven Year Unsecured Senior Notes Three Point Nine Five Four Percent
Debt Instrument [Line Items]
Coupon/Stated Rate	3.85%
Effective Rate	3.95%	[1]
Principal Amount	1,000,000
Maturity date	1-Feb-23	[2]
Senior Notes [Member] | Ten and Half Year Unsecured Senior Notes Three Point Two Seven Nine Percent [Member]
Debt Instrument [Line Items]
Coupon/Stated Rate	3.13%
Effective Rate	3.28%	[1]
Principal Amount	500,000
Maturity date	1-Sep-23	[2]
Senior Notes [Member] | Ten and Half Year Unsecured Senior Notes Three Point Nine One Six Percent [Member]
Debt Instrument [Line Items]
Coupon/Stated Rate	3.80%
Effective Rate	3.92%	[1]
Principal Amount	$700,000
Maturity date	1-Feb-24	[2]
[1]	Yield on issuance date including the effects of discounts on the notes and the amortization of financing costs.
[2]	principal amounts are due prior to maturity.

Unsecured_Senior_Notes_Unsecur

Unsecured Senior Notes Unsecured Senior Notes (Narrative) (Details) (USD $)	12 Months Ended			0 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 15, 2014
Debt Instrument [Line Items]
Repayments of Unsecured Debt	$548,016,000	$0	$224,261,000
Proceeds from unsecured senior notes	0	1,194,753,000	997,790,000
Maximum Secured Debt Leverage Ratio	55.00%
Losses from early extinguishments of debt	-10,633,000	122,000	-4,453,000
Senior Notes [Member]
Debt Instrument [Line Items]
Payment Of debt redemption premium				10,500,000
Maximum Leverage Ratio	60.00%
Maximum Secured Debt Leverage Ratio	50.00%
Minimum Interest Coverage Ratio	1.5
Minimum Unencumbered Asset Value Percentage Of Unsecured Debt	150.00%
Losses from early extinguishments of debt				-10,600,000
Senior Notes [Member] | Five Percent Unsecured Senior Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount				250,000,000
Coupon/Stated Rate				5.00%
Repayments of Unsecured Debt				255,800,000
Debt Instrument, Increase, Accrued Interest				500,000
Debt Instrument, Redemption Price, Percentage				102.13%
Senior Notes [Member] | Five Point Six Two Five Percent Unsecured Senior Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount				300,000,000
Coupon/Stated Rate				5.63%
Repayments of Unsecured Debt				308,000,000
Debt Instrument, Increase, Accrued Interest				$2,800,000
Debt Instrument, Redemption Price, Percentage				101.73%

Unsecured_Exchangeable_Senior_1

Unsecured Exchangeable Senior Notes (Narrative) (Details) (USD $)	12 Months Ended			0 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Feb. 18, 2014
Debt Instrument [Line Items]
Unsecured exchangeable senior notes	$0	$744,880,000
Net Of Adjustment For Equity Component Allocation	0	2,400,000
Debt Instrument Convertible Initial Carrying Amount Of The Equity Component	230,300,000
Deferred Financing Costs Equity Component	1,000,000
Debt Instrument, Convertible, Carrying Amount of Equity Component	0	91,900,000
Debt Instrument, Convertible, Interest Expense	3,300,000	34,800,000	48,400,000
Interest Expense Adjustment Related To Convertible Debt	2,400,000	23,100,000	29,100,000
Amount paid for repurchase	548,016,000	0	224,261,000
Common stock, shares issued	153,192,845	153,062,001
(Losses) gains from early extinguishments of debt	-10,633,000	122,000	-4,453,000
3.750% Exchangeable Senior Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount		450,000,000
Coupon/Stated Rate		3.75%
Convertible Debt Effective Interest Rate		5.96%
Three point six two five percent exchangeable senior notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount	747,500,000
Coupon/Stated Rate	3.63%
Convertible Debt Effective Interest Rate	6.56%
2.875% Exchangeable Senior Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount			862,500,000
Coupon/Stated Rate			2.88%
Convertible Debt Effective Interest Rate			5.63%
Three point six two five percent exchangeable senior notes [Member] | Unsecured Debt [Member]
Debt Instrument [Line Items]
Coupon/Stated Rate				3.63%
Amount paid for repurchase				$747,500,000

Unsecured_Line_of_Credit_Detai

Unsecured Line of Credit (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
	Y
Line of Credit Facility [Line Items]
Line of Credit Facility, Current Borrowing Capacity	$1,000,000,000
Line of Credit Facility, Expiration Date	26-Jul-18
Line of Credit Facility, Frequency of Commitment Fee Payment	quarterly
Line of Credit Facility, Amount Outstanding	0	0
Leverage Ratio, Maximum	60.00%
Leverage Ratio Maximum Increasing Limit	65.00%
Leverage Ratio Reduction Limit	60.00%
Leverage Ratio Reduction Period, Years	1
Maximum Secured Debt Leverage Ratio	55.00%
Fixed Charge Coverage Ratio Minimum.	1.4
Maximum Unsecured Debt Leverage Ratio	60.00%
Unsecured Debt Leverage Ratio Maximum Increasing Limit	65.00%
Unsecured Debt Leverage Ratio Reduction Limit	60.00%
Unsecured Debt Leverage Ratio Reduction Period, Years	1
Unsecured Debt Interest Coverage Ratio Minimum.	1.75
Alternative Base Interest Rate Calculation [Member]
Line of Credit Facility [Line Items]
Amount added to Federal Funds Rate to Calculate Interest Rate	0.50%
LIBOR Period Used to Calculate Interest Rate (in months)	1
Amount added to LIBOR to Calculate Interest Rate	1.00%
Maximum [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Current Borrowing Capacity	$1,500,000,000
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage	0.35%
Maximum [Member] | Interest Rate Based on LIBOR or CDOR [Member]
Line of Credit Facility [Line Items]
Margin added to Calculated Interest Rate	1.70%
Maximum [Member] | Alternative Base Interest Rate Calculation [Member]
Line of Credit Facility [Line Items]
Margin added to Calculated Interest Rate	0.70%
Minimum [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage	0.13%
Minimum [Member] | Interest Rate Based on LIBOR or CDOR [Member]
Line of Credit Facility [Line Items]
Margin added to Calculated Interest Rate	0.93%
Minimum [Member] | Alternative Base Interest Rate Calculation [Member]
Line of Credit Facility [Line Items]
Margin added to Calculated Interest Rate	0.00%
Rate Based on Current Credit Rating [Member] | Interest Rate Based on LIBOR or CDOR [Member]
Line of Credit Facility [Line Items]
Margin added to Calculated Interest Rate	1.00%
Rate Based on Current Credit Rating [Member] | Alternative Base Interest Rate Calculation [Member]
Line of Credit Facility [Line Items]
Margin added to Calculated Interest Rate	0.00%
Rate Based on Current Credit Rating [Member] | Current Rates Based on Credit Ratings [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage	0.15%

Commitments_And_Contingencies_

Commitments And Contingencies (Details) (USD $)	5 Months Ended	7 Months Ended	10 Months Ended	12 Months Ended		5 Months Ended
	Dec. 31, 2014	Jul. 28, 2014	Oct. 22, 2014	Dec. 31, 2014	Dec. 31, 2009	Jun. 09, 2014	Sep. 18, 2014	Jan. 10, 2014
	Buildings			Buildings
Commitments And Contingencies [Line Items]
Letter of credit and performance obligations	$19,800,000			$19,800,000
Property insurance program per occurrence limits	1,000,000,000			1,000,000,000
Per occurrence limit for NBCR Coverage				1,000,000,000
Value of program trigger	100,000,000			100,000,000
Coinsurance of program trigger				15.00%
Deductible in insurance as a percentage of the value of the affected property, San Francisco				5.00%
Per occurrence limit of the earthquake insurance which covers San Francisco region				120,000,000
Annual aggregate limit of the earthquake insurance which covers San Francisco region				120,000,000
Amount of earthquake insurance provided by IXP, LLC as direct insurer San Francisco				20,000,000
Earthquake Coverage Included In Builders Risk Policy For 535 Mission			15,000,000
Earthquake Coverage Included In Builders Risk Policy For Below Grade improvements for Salesforce Tower	60,000,000	15,000,000
Operating partnership guarantee to cover liabilities of IXP	20,000,000			20,000,000
Number of Insurer(s)	1			1
Contractual Tax Agreements Number Of Properties	3			3
Expiration of Tax Protection for the General Motors Buidling				9-Jun-17
Lehman [Member]
Commitments And Contingencies [Line Items]
Bankruptcy claim, amount filed by general creditor					45,300,000
Bankruptcy claim amount allowed by court to creditor	37,500,000			37,500,000				45,200,000
Bankruptcy Claims, Amount of Claims Settled							7,700,000
Bankruptcy claim, denominator used to evaluate claim price amount				1
Minimum [Member] | Lehman [Member]
Commitments And Contingencies [Line Items]
Bankruptcy claim, price on the private market				0.24
Maximum [Member] | Lehman [Member]
Commitments And Contingencies [Line Items]
Bankruptcy claim, price on the private market				0.25
Fountain Square [Member]
Commitments And Contingencies [Line Items]
Guarantee Obligation Related To Tenant Re-Leasing Costs				700,000
Maturity Date				11-Oct-16
John Hancock Tower [Member]
Commitments And Contingencies [Line Items]
Guarantee Obligation Related To Tenant Re-Leasing Costs				25,700,000
Maturity Date				6-Jan-17
601 Lexington Avenue [Member]
Commitments And Contingencies [Line Items]
Coverage For Acts Of Terrorism Under TRIA Covered in Excess of Amount Covered by IXP				250,000,000
767 Venture, LLC [Member]
Commitments And Contingencies [Line Items]
Maximum funding obligation	32,000,000			32,000,000
Property insurance program per occurrence limits	1,625,000,000			1,625,000,000
Coverage for acts of Terrorism certified under TRIA in excess of amount provided by NYXP, LLC, as a direct insurer						1,375,000,000
Coverage for acts of terrorism certified under TRIA covered by NYXP						$250,000,000

Noncontrolling_Interests_Narra

Noncontrolling Interests (Narrative) (Details)	Dec. 31, 2014
Noncontrolling Interests [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
OP Units	16,453,670
Long Term Incentive Plan Units	1,496,799
Series Four Preferred Units	12,667
OPP Units 2012 [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Outperformance Awards Long Term Incentive Plan Units	394,590
MYLTIP 2013 [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Outperformance Awards Long Term Incentive Plan Units	313,936
MYLTIP 2014 [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Outperformance Awards Long Term Incentive Plan Units	482,032

Noncontrolling_Interests_Redee

Noncontrolling Interests (Redeemable Preferred Units) (Narrative) (Details) (USD $)

0 Months Ended

12 Months Ended

12-May-14

Dec. 31, 2013

Dec. 31, 2014

Feb. 18, 2014

Nov. 17, 2014

Nov. 05, 2014

Oct. 16, 2014

Sep. 30, 2014

Aug. 15, 2014

Jul. 03, 2014

19-May-14

15-May-14

Aug. 29, 2013

D

Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]

Redeemable preferred units of the Operating Partnership

51,312,000

$633,000

Noncontrolling Interests [Member]

Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]

Series Four Preferred Units

12,667

Series Two Preferred Units [Member] | Noncontrolling Interests [Member]

Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]

Series Two Preferred Units (in shares)

666,116

Preferred Units Liquidation Preference Percentage

6.00%

Operating Units liquidation preference (in dollars per share)

$38.10

OP Units for each Preferred Unit (in shares)

1.312336

Units converted (in shares)

329,881

Series Two Preferred Units On Converted Basis

874,168

432,914

Cash Distributions To Preferred Unitholders Per Unit

$0.85

Preferred Units Liquidation Preference

$50

Series Four Preferred Units [Member]

Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]

Series Four Preferred Units Annual Dividend Payable Rate

2.00%

Preferred Units Liquidation Preference

$50

Series Four Preferred Units [Member] | Noncontrolling Interests [Member]

Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]

Series Four Preferred Units Redeemed

861,400

Cash Distributions To Preferred Unitholders Per Unit

$0.25

$0.25

$0.25

$0.25

Series Four Preferred Units Annual Dividend Payable Rate

2.00%

Preferred Units Liquidation Preference

$50

$50

$50

First opportunity to redeem Series Four Preferred Units (in days)

30

Series Four Preferred Units Released from Escrow

27,773

319,687

Redemption of Series Four Preferred Units

$1,400,000

$16,000,000

Noncontrolling_Interests_Redee1

Noncontrolling Interests (Redeemable Preferred Units) (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Stockholders' Equity Attributable to Noncontrolling Interest [Roll Forward]
Beginning Balance	$51,312
Net income	1,023	6,046	3,497
Redemption of redeemable preferred units (Series Four Preferred Units)	-17,373	-43,070	-18,329
Conversion of redeemable preferred units (Series Two Preferred Units) to common units	33,306	16,494	5,852
Ending Balance	633	51,312
Redeemable Preferred Units [Member]
Stockholders' Equity Attributable to Noncontrolling Interest [Roll Forward]
Beginning Balance	51,312	110,876	55,652
Issuance of redeemable preferred units (Series Four Preferred Units)			79,405
Net income	1,023	6,046	3,497
Distributions	-1,023	-6,046	-3,497
Redemption of redeemable preferred units (Series Four Preferred Units)	-17,373	-43,070	-18,329
Conversion of redeemable preferred units (Series Two Preferred Units) to common units	-33,306	-16,494	-5,852
Ending Balance	$633	$51,312	$110,876

Noncontrolling_Interests_Redee2

Noncontrolling Interests Redeemable interest in property partnerships (narrative) (Details) (Consolidated Properties [Member], Fountain Square [Member], Noncontrolling Interests [Member], USD $)	0 Months Ended
	Oct. 04, 2012
Consolidated Properties [Member] | Fountain Square [Member] | Noncontrolling Interests [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Aggregate purchase price	$385,000,000
Debt recorded upon consolidation	211,300,000
Ownership Percentage	50.00%
Consideration paid	87,000,000
Ownership Percentage by Parent	50.00%
Future Fixed Price to Acquire Remaining Portion of Joint venture	$102,000,000
Fixed Price Option Expiration Date	31-Jan-16

Noncontrolling_Interests_Redee3

Noncontrolling Interests Redeemable Property Partnerships (Details) (USD $)	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013	Dec. 31, 2012
Redeemable Property Partnership [Abstract]
Misstatement of real estate operating expense	$1,900,000
Stockholders' Equity Attributable to Noncontrolling Interest [Roll Forward]
Beginning Balance	99,609,000		97,558,000	0
Acquisition-date fair value of redeemable interest				98,787,000
Net loss	-603,000		-1,839,000	-719,000
Distributions	-6,000,000		-4,585,000	-3,032,000
Adjustment to reflect redeemable interest at redemption value	11,686,000	[1]	8,475,000	2,522,000
Ending Balance	$104,692,000		$99,609,000	$97,558,000
[1]	(1) Includes an out-of-period adjustment totaling approximately $1.9 million (See Note 2).

Common_Units_Narrative_Details

(Common Units) (Narrative) (Details) (USD $)	0 Months Ended				12 Months Ended		9 Months Ended
	Oct. 31, 2014	Jul. 31, 2014	Apr. 30, 2014	Jan. 29, 2014	Dec. 31, 2014	Dec. 31, 2013	Sep. 30, 2014	Dec. 08, 2014
					Y
OP Units [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Distributions Declared To OP And LTIP Units Per Unit								$0.65
Distributions Declared To OPP and MYLTIP Units Per Unit								$0.07
OP Units for redemption (in shares)					80,246	929,441
Redemption of OP units issued on conversion of Series Two Preferred Units (in shares)					3,734	432,914
Redemption of OP units issued on conversion of LTIP Units (in shares)					67,857	24,028
Outperformance Awards LTIP Units distribution as percentage of OP Unit distribution prior to measurement date					10.00%
Distributions made to OP and LTIP units per unit (in dollars per share)	$0.65	$0.65	$0.65	$0.65
Distribution paid to outperformance awards, OPP Units and MYLTIP Units (in dollars per share)	$0.07	$0.07	$0.07	$0.07
Restriction on redemption of OP Unit to Common Stock (in years)					1
Redemption of OP Unit equivalence to Common Stock (in shares)					1
Common units of operating partnership if converted value					$2,300,000,000
OPP Units 2012 [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Outperformance awards in LTIP Units (in shares)					394,590
MYLTIP 2013 [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Outperformance awards in LTIP Units (in shares)					313,936
OP Units [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Closing price of common stock (in dollars per share)					$128.69
Quarter 3 2014 [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Dividend payable date					31-Oct-14
Quarter 3 2014 [Member] | OP Units [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Distribution Made to Limited Partner, Date of Record					30-Sep-14
Dividend payable date					31-Oct-14
Quarter 4 2013 [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Dividend payable date					29-Jan-14
Quarter 4 2013 [Member] | OP Units [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Distribution Made to Limited Partner, Date of Record							31-Dec-13
Dividend payable date					29-Jan-14
Quarter 1 2014 [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Dividend payable date					30-Apr-14
Quarter 1 2014 [Member] | OP Units [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Distribution Made to Limited Partner, Date of Record					31-Mar-14
Dividend payable date					30-Apr-14
Quarter 2 2014 [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Dividend payable date					31-Jul-14
Quarter 2 2014 [Member] | OP Units [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Distribution Made to Limited Partner, Date of Record					30-Jun-14
Dividend payable date					31-Jul-14
Quarter 4 2014 [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Dividend payable date					28-Jan-15
Dividends Payable, Date Declared					8-Dec-14
Quarter 4 2014 [Member] | OP Units [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Distribution Made to Limited Partner, Date of Record					31-Dec-14
Dividend payable date					28-Jan-15
Dividends Payable, Date Declared					8-Dec-14
special dividend [Member] | OP Units [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Distributions Declared To OP And LTIP Units Per Unit								$4.50
Distributions made to OP and LTIP units per unit (in dollars per share)				$2.25
OPP Units 2011 [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Accelerated Compensation Expenses Related To 2011 OPP Units					$1,200,000

Noncontrolling_Interests_Prope

Noncontrolling Interests (Property Partnerships) (Narrative) (Details) (USD $)	12 Months Ended			0 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	31-May-13	Apr. 10, 2014	Feb. 26, 2013	Oct. 30, 2014	Oct. 09, 2013	Feb. 07, 2014	Nov. 05, 2014	Jul. 03, 2014
				partners	floor		floor	sqft
				sqft	sqft
Noncontrolling Interest [Line Items]
Noncontrolling interest in property partnerships recorded upon consolidation	$0	$480,861,000	$0
Property partnerships	1,602,467,000	726,132,000
767 5th Avenue (The General Motors Building) [Member]
Noncontrolling Interest [Line Items]
Net Rentable Area				1,800,000
Number of Joint Venture Partners				2
Noncontrolling interest in property partnerships recorded upon consolidation				480,861,000
767 5th Avenue (The General Motors Building) [Member] | Consolidated Properties [Member]
Noncontrolling Interest [Line Items]
Partners Joint Venture Ownership Percentage				40.00%
Noncontrolling interest in property partnerships recorded upon consolidation				480,900,000
767 5th Avenue (The General Motors Building) [Member]
Noncontrolling Interest [Line Items]
Number of Joint Venture Partners				2
Noncontrolling Interests [Member]
Noncontrolling Interest [Line Items]
Noncontrolling interest in property partnerships recorded upon consolidation		480,861,000
Noncontrolling Interests [Member] | Consolidated Properties [Member]
Noncontrolling Interest [Line Items]
Net Rentable Area								1,246,000
Number of Joint Venture Partners				2
Noncontrolling Interests [Member] | 601 Lexington Avenue, Atlantic Wharf Office Building and 100 Federal Street [Member] | Consolidated Properties [Member]
Noncontrolling Interest [Line Items]
Ownership Percentage							45.00%
Sale Price Of Sold Property							1,827,000,000
Disposal Group, Including Discontinued Operation, Secured Debt							712,900,000
Proceeds from sale of an interest in a real estate asset							1,497,000,000
Difference between net cash proceeds received and noncontrolling interest recognized							648,400,000
Noncontrolling Interests [Member] | Time Square Tower [Member] | Consolidated Properties [Member]
Noncontrolling Interest [Line Items]
ownership interest sold (percent)								45.00%
Net Contractual Sales Price								684,000,000
Proceeds from sale of an interest in a real estate asset								673,100,000
Portion allocation to noncontrolling interest based on carrying value of the total equity of the property prior to the sale								243,500,000
Difference between net cash proceeds received and noncontrolling interest recognized								429,600,000
Salesforce Tower [Member] | Consolidated Properties [Member]
Noncontrolling Interest [Line Items]
Ownership Percentage by Parent					95.00%
Number of floors in building					61
Area of Real Estate Property					1,400,000
Salesforce Tower [Member] | Noncontrolling Interests [Member] | Consolidated Properties [Member]
Noncontrolling Interest [Line Items]
maximum percentage of construction costs to allow partner to have a loan						50.00%
Ownership Percentage						5.00%			50.00%
Ownership Percentage by Parent						95.00%
Portion of total costs partner can require the company to fund						2.50%
portion of partners equity that can be funded by the company						50.00%
One Hundred Federal Street [Member] | Noncontrolling Interests [Member] | 601 Lexington Avenue, Atlantic Wharf Office Building and 100 Federal Street [Member] | Consolidated Properties [Member]
Noncontrolling Interest [Line Items]
Area of Real Estate Property							1,323,000
601 Lexington Avenue [Member] | Noncontrolling Interests [Member] | 601 Lexington Avenue, Atlantic Wharf Office Building and 100 Federal Street [Member] | Consolidated Properties [Member]
Noncontrolling Interest [Line Items]
Area of Real Estate Property							1,669,000
Atlantic Wharf Office [Member] | Noncontrolling Interests [Member] | 601 Lexington Avenue, Atlantic Wharf Office Building and 100 Federal Street [Member] | Consolidated Properties [Member]
Noncontrolling Interest [Line Items]
Area of Real Estate Property							791,000
Low Rise [Member] | 601 Lexington Avenue [Member] | Noncontrolling Interests [Member] | 601 Lexington Avenue, Atlantic Wharf Office Building and 100 Federal Street [Member] | Consolidated Properties [Member]
Noncontrolling Interest [Line Items]
Number of floors in building							6
Office Tower [Member] | 601 Lexington Avenue [Member] | Noncontrolling Interests [Member] | 601 Lexington Avenue, Atlantic Wharf Office Building and 100 Federal Street [Member] | Consolidated Properties [Member]
Noncontrolling Interest [Line Items]
Number of floors in building							59
Series Four Preferred Units [Member] | Noncontrolling Interests [Member]
Noncontrolling Interest [Line Items]
Redemption of Series Four Preferred Units										$1,400,000	$16,000,000

Stockholders_Equity_Details

Stockholders' Equity (Details) (USD $)

0 Months Ended

12 Months Ended

0 Months Ended

7 Months Ended

0 Months Ended

Oct. 31, 2014

Jul. 30, 2014

Apr. 30, 2014

Jan. 29, 2014

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2012

Jun. 03, 2014

Dec. 31, 2014

Nov. 17, 2014

Aug. 15, 2014

15-May-14

Feb. 18, 2014

Dec. 08, 2014

Jun. 02, 2014

Mar. 27, 2018

Y

Class of Stock [Line Items]

Common stock, shares outstanding

153,113,945

152,983,101

153,113,945

Shares of Common Stock upon the exercise of options to purchase Common Stock by certain employees

21,459

0

22,823

Shares of its Common Stock in connection with the redemption of an equal number of OP Units

80,246

929,441

Dividends Payable, Amount Per Share

$0.65

Dividend paid on common stock (in dollars per share)

$0.65

$0.65

$0.65

$2.90

Preferred stock, shares outstanding

0

0

0

Proceeds from Issuance of Preferred Stock and Preference Stock

$0

$193,623,000

$0

New ATM program [Member]

Class of Stock [Line Items]

At the market stock offering program, aggregate value of common stock

600,000,000

At Market Stock Offering Program Maximum Length Of Sale In Years

3

ATM program amount issued

0

Atm Program [Member]

Class of Stock [Line Items]

At the market stock offering program, aggregate value of common stock

600,000,000

At the market stock offering program remaining amount available for issuance

$305,300,000

Quarter 3 2014 [Member]

Class of Stock [Line Items]

Dividend payable date

31-Oct-14

Dividend shareholder of record date

30-Sep-14

Quarter 4 2013 [Member]

Class of Stock [Line Items]

Dividend payable date

29-Jan-14

Dividend shareholder of record date

31-Dec-13

Quarter 4 2014 [Member]

Class of Stock [Line Items]

Dividend payable date

28-Jan-15

Dividend shareholder of record date

31-Dec-14

Dividends Payable, Date Declared

8-Dec-14

Quarter 1 2014 [Member]

Class of Stock [Line Items]

Dividend payable date

30-Apr-14

Dividend shareholder of record date

31-Mar-14

Quarter 2 2014 [Member]

Class of Stock [Line Items]

Dividend payable date

31-Jul-14

Dividend shareholder of record date

30-Jun-14

Quarter 1 2015 [Member]

Class of Stock [Line Items]

Dividend payable date

17-Feb-15

Dividends Payable, Date Declared

8-Dec-14

Series B Cumulative Redeemable Preferred Stock [Member]

Class of Stock [Line Items]

Dividends Payable, Amount Per Share

$32.81

Dividend paid on common stock (in dollars per share)

$32.81

$32.81

$32.81

$32.81

Preferred stock, shares outstanding

80,000

80,000

80,000

Preferred Stock, Dividend Rate, Percentage

5.25%

Series B Liquidation Preference Per Share

$2,500

$2,500

$2,500

Series B Cumulative Redeemable Preferred Stock [Member] | Quarter 3 2014 [Member]

Class of Stock [Line Items]

Dividend payable date

15-Aug-14

Series B Cumulative Redeemable Preferred Stock [Member] | Quarter 4 2014 [Member]

Class of Stock [Line Items]

Dividend payable date

17-Nov-14

Series B Cumulative Redeemable Preferred Stock [Member] | Quarter 1 2014 [Member]

Class of Stock [Line Items]

Dividend payable date

18-Feb-14

Series B Cumulative Redeemable Preferred Stock [Member] | Quarter 2 2014 [Member]

Class of Stock [Line Items]

Dividend payable date

15-May-14

Series B Cumulative Redeemable Preferred Stock [Member] | Quarter 1 2015 [Member]

Class of Stock [Line Items]

Dividend shareholder of record date

5-Feb-15

Depositary shares of Series B Cumulative Redeemable Preferred [Member]

Class of Stock [Line Items]

Preferred stock, shares outstanding

8,000,000

8,000,000

Series B Liquidation Preference Per Share

$25

$25

$25

special dividend [Member]

Class of Stock [Line Items]

Dividends Payable, Amount Per Share

$4.50

Subsequent Event [Member] | Series B Cumulative Redeemable Preferred Stock [Member]

Class of Stock [Line Items]

Series B Liquidation Preference Per Share

$2,500

Future_Minimum_Rents_Schedule_

Future Minimum Rents Schedule of Future Minimum Rents Payable for Operating Leases (Details) (USD $)	Dec. 31, 2014
In Thousands, unless otherwise specified
Future Minimum Rents [Abstract]
2015	$1,796,517
2016	1,788,978
2017	1,677,005
2018	1,568,200
2019	1,483,188
Thereafter	$9,097,899

Future_Minimum_Rents_Narrative

Future Minimum Rents Narrative (Details)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
	tenants	tenants	tenants
Future Minimum Rent [Line Items]
Number Of Tenants Above Concentration Amount.	0	0	0
Concentration Risk, Percentage	10.00%	10.00%	10.00%
Minimum [Member]
Future Minimum Rent [Line Items]
Initial Term Lease Expiration Date	2015
Maximum [Member]
Future Minimum Rent [Line Items]
Initial Term Lease Expiration Date	2046

Segment_Information_Schedule_O

Segment Information (Schedule Of Segment Reporting By Geographic Area And Property Type) (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	$2,240,119	$1,987,671	$1,715,425
Rental Revenue: Office/Technical	61,985	55,525	39,218
Rental Revenue: Residential	26,193	22,318	20,568
Rental Revenue: Hotel	43,385	40,330	37,915
Rental Revenue Total	2,371,682	2,105,844	1,813,126
Rental Revenue: % of Grand Totals	100.00%	100.00%	100.00%
Rental Expenses: Class A Office	802,902	716,049	617,482
Rental Expenses: Office/Technical	16,466	14,777	10,614
Rental Expenses: Residential	15,922	12,130	10,992
Rental Expenses: Hotel	29,236	28,447	28,120
Rental Expenses: Total	864,526	771,403	667,208
Rental Expenses: % Of Grand Totals	100.00%	100.00%	100.00%
Net operating Income	1,507,156	1,334,441	1,145,918
Net operating Income: % of Grand Totals	100.00%	100.00%	100.00%
Boston [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	692,116	665,991	617,652
Rental Revenue: Office/Technical	23,801	22,617	22,460
Rental Revenue: Residential	4,528	4,395	3,936
Rental Revenue: Hotel	43,385	40,330	37,915
Rental Revenue Total	763,830	733,333	681,963
Rental Revenue: % of Grand Totals	32.21%	34.82%	37.61%
Rental Expenses: Class A Office	270,947	259,997	242,904
Rental Expenses: Office/Technical	7,173	6,879	6,499
Rental Expenses: Residential	1,957	1,823	1,675
Rental Expenses: Hotel	29,236	28,447	28,120
Rental Expenses: Total	309,313	297,146	279,198
Rental Expenses: % Of Grand Totals	35.78%	38.52%	41.85%
Net operating Income	454,517	436,187	402,765
Net operating Income: % of Grand Totals	30.16%	32.69%	35.15%
New York [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	928,692	725,566	543,194
Rental Revenue: Office/Technical	0	0	0
Rental Revenue: Residential	0	0	0
Rental Revenue: Hotel	0	0	0
Rental Revenue Total	928,692	725,566	543,194
Rental Revenue: % of Grand Totals	39.16%	34.46%	29.96%
Rental Expenses: Class A Office	315,330	251,640	187,987
Rental Expenses: Office/Technical	0	0	0
Rental Expenses: Residential	0	0	0
Rental Expenses: Hotel	0	0	0
Rental Expenses: Total	315,330	251,640	187,987
Rental Expenses: % Of Grand Totals	36.47%	32.62%	28.18%
Net operating Income	613,362	473,926	355,207
Net operating Income: % of Grand Totals	40.70%	35.51%	31.00%
San Francisco [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	237,381	214,755	208,177
Rental Revenue: Office/Technical	23,840	17,259	494
Rental Revenue: Residential	0	0	0
Rental Revenue: Hotel	0	0	0
Rental Revenue Total	261,221	232,014	208,671
Rental Revenue: % of Grand Totals	11.01%	11.02%	11.51%
Rental Expenses: Class A Office	85,178	77,905	75,542
Rental Expenses: Office/Technical	4,955	3,708	149
Rental Expenses: Residential	0	0	0
Rental Expenses: Hotel	0	0	0
Rental Expenses: Total	90,133	81,613	75,691
Rental Expenses: % Of Grand Totals	10.43%	10.58%	11.34%
Net operating Income	171,088	150,401	132,980
Net operating Income: % of Grand Totals	11.35%	11.27%	11.60%
Washington, DC [Member]
Segment Reporting Information [Line Items]
Rental Revenue: Class A Office	381,930	381,359	346,402
Rental Revenue: Office/Technical	14,344	15,649	16,264
Rental Revenue: Residential	21,665	17,923	16,632
Rental Revenue: Hotel	0	0	0
Rental Revenue Total	417,939	414,931	379,298
Rental Revenue: % of Grand Totals	17.62%	19.70%	20.92%
Rental Expenses: Class A Office	131,447	126,507	111,049
Rental Expenses: Office/Technical	4,338	4,190	3,966
Rental Expenses: Residential	13,965	10,307	9,317
Rental Expenses: Hotel	0	0	0
Rental Expenses: Total	149,750	141,004	124,332
Rental Expenses: % Of Grand Totals	17.32%	18.28%	18.63%
Net operating Income	$268,189	$273,927	$254,966
Net operating Income: % of Grand Totals	17.79%	20.53%	22.25%

Segment_Information_Schedule_O1

Segment Information (Schedule Of Reconciliation Of Net Operating Income To Net Income) (Details) (USD $)	3 Months Ended								12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Development and management services income									$25,316	$29,695	$34,060
Income from unconsolidated joint ventures									12,769	75,074	49,078
Gains on consolidation of joint ventures									0	385,991	0
Interest and other income									8,765	8,310	10,091
Gains from investments in securities									1,038	2,911	1,389
(Losses) gains from early extinguishments of debt									-10,633	122	-4,453
Income from discontinued operations									0	8,022	9,806
Gains on sales of real estate from discontinued operations									0	112,829	36,877
Gain on forgiveness of debt from discontinued operations									0	20,182	0
Gains on sales of real estate									168,039	0	0
General and administrative expense									98,937	115,329	90,129
Transaction costs									3,140	1,744	3,653
Depreciation and amortization expense									628,573	560,637	445,875
Interest expense									455,743	446,880	410,970
Impairment loss									0	8,306	0
Impairment loss from discontinued operations									0	3,241	0
Noncontrolling interest in property partnerships									30,561	1,347	3,792
Noncontrolling interest-redeemable preferred units of the Operating Partnership									1,023	6,046	3,497
Noncontrolling interest-common units of the Operating Partnership									50,862	70,085	30,125
Noncontrolling interest in discontinued operations-common units of the Operating Partnership									0	14,151	5,075
Preferred dividends									10,500	8,057	0
Net income attributable to Boston Properties, Inc. common shareholders	174,510	127,724	76,527	54,034	88,719	152,677	452,417	47,854	433,111	741,754	289,650
Business Intersegment, Eliminations [Member]
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Net Operating Income									1,507,156	1,334,441	1,145,918
Development and management services income									25,316	29,695	34,060
Income from unconsolidated joint ventures									12,769	75,074	49,078
Gains on consolidation of joint ventures									0	385,991	0
Interest and other income									8,765	8,310	10,091
Gains from investments in securities									1,038	2,911	1,389
(Losses) gains from early extinguishments of debt									-10,633	122	-4,453
Income from discontinued operations									0	8,022	9,806
Gains on sales of real estate from discontinued operations									0	112,829	36,877
Gain on forgiveness of debt from discontinued operations									0	20,182	0
Gains on sales of real estate									168,039	0	0
General and administrative expense									98,937	115,329	90,129
Transaction costs									3,140	1,744	3,653
Depreciation and amortization expense									628,573	560,637	445,875
Interest expense									455,743	446,880	410,970
Impairment loss									0	8,306	0
Impairment loss from discontinued operations									0	3,241	0
Noncontrolling interest in property partnerships									30,561	1,347	3,792
Noncontrolling interest-redeemable preferred units of the Operating Partnership									1,023	6,046	3,497
Noncontrolling interest-common units of the Operating Partnership									50,862	70,085	30,125
Noncontrolling interest in discontinued operations-common units of the Operating Partnership									0	14,151	5,075
Preferred dividends									10,500	8,057	0
Net income attributable to Boston Properties, Inc. common shareholders									$433,111	$741,754	$289,650

Segment_Information_Segment_re

Segment Information Segment reporting (narrative) (Details) (767 5th Avenue (The General Motors Building) [Member])	0 Months Ended
	31-May-13
	partners
767 5th Avenue (The General Motors Building) [Member]
Segment Reporting Information [Line Items]
Number of Joint Venture Partners	2

Earnings_Per_Share_Details

Earnings Per Share (Details) (USD $)	3 Months Ended								12 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Basic Earnings:
Income from continuing operations attributable to Boston Properties, Inc., Income (Numerator)										$618,113	$248,042
Income (loss) from continuing operations before discontinued operations and allocation of undistributed earnings (share)										152,201	150,120
Income (Loss) from Continuing Operations, Per Basic Share									$2.83	$4.06	$1.65
Net income attributable to Boston Properties Inc., Shares (Denominator)									153,089	152,201	150,120
Discontinued operations attributable to Boston Properties, Inc., Income (Numerator)										123,641	41,608
Discontinued Operations (in shares)										0	0
Discontinued operations attributable to Boston Properties, Inc., Per Share Amount (in dollars per share)									$0	$0.81	$0.28
Other Preferred Stock Dividends and Adjustments										-160
Other Preferred Stock Dividends And Adjustments (in shares)										0
Other Preferred Stock Dividends And Adjustments (per share)										$0
Net Income Loss Available To Boston Properties, Inc. Common Stockholders									433,111	741,594	289,650
Net income	$1.14	$0.83	$0.50	$0.35	$0.58	$1	$2.95	$0.32	$2.83	$4.87	$1.93
Effect of Dilutive Securities:
Stock Based Compensation and Exchangeable Senior Notes, Income (Numerator)									0	0	0
Stock Based Compensation and Exchangeable Senior Notes, Shares (Denominator)									219	320	591
Stock Based Compensation and Exchangeable Senior Notes Per Share Amount									$0	($0.01)	($0.01)
Diluted Earnings:
Diluted Earnings: Net income, Income (Numerator)									$433,111	$741,594	$289,650
Diluted Earnings: Net income, Shares (Denominator)									153,308	152,521	150,711
Net income	$1.14	$0.83	$0.50	$0.35	$0.58	$1	$2.94	$0.31	$2.83	$4.86	$1.92

Employee_Benefit_Plans_Details

Employee Benefit Plans (Details) (USD $)	12 Months Ended			168 Months Ended	180 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2014	Dec. 31, 2014
	M			age
				Y
Employee Benefit Plans [Abstract]
Eligibility Period For 401K Savings Plan In Months	3
Employer Matching Contribution, Percent of Match					200.00%
Employer Matching Contribution, Percent of Employees' Gross Pay					3.00%
Maximum Eligible Earnings As Established By IRS For Matching Contributions	$260,000	$255,000	$250,000
Employer Contribution Amount	3,500,000	3,400,000	3,200,000
Years Of Service For Supplemental Retirement Contribution				10
Age Of Employee For Supplemental Retirement Contribution				40
Annual Supplemental Retirement Contribution And Credit Total.	52,000	60,000	78,000
Separate Unrestricted Cash For Deferred Compensation Plan.	19,500,000	16,600,000		19,500,000	19,500,000
Deferred Compensation Liability	$19,500,000	$16,600,000		$19,500,000	$19,500,000

Stock_Option_and_Incentive_Pla2

Stock Option and Incentive Plan (Narrative) (Details) (USD $)	0 Months Ended		12 Months Ended
	15-May-12	Jan. 27, 2014	Dec. 31, 2014	Dec. 31, 2013	Apr. 02, 2013	Mar. 11, 2013
	Y	indices
		tiers
		Y
Two Thousand Twelve Stock Option and Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Additional Shares Authorized	13,000,000
Other Than Stock Options Awards Conversion Ratio	2.32
Stock Option Awards Conversion Ratio	1
Stock Option and Incentive Plan Term Period	10
MYLTIP 2013 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
MYLTIP units issued				318,926
MYLTIP 2014 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of Tiers		4
TRS measurement (in years)		3
Distributions percent before measurement date		10.00%
Indices Used to Compare TRS		2
Percentage of annualized TRS for Reduction of Earned Awards		0.00%
Percentage to Cause Some Awards to be Earned Even if on a Relative Basis it Would Not Result in any Earned Awards		12.00%
MYLTIP units issued			485,459
Value of MYLTIP Awards		$12,700,000
MYLTIP Value Amortized Into Earnings, Years		4
OPP Units 2011 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Accelerated Compensation Expenses Related To 2011 OPP Units			1,200,000
Exceptional [Member] | MYLTIP 2014 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Amount of Each Tier		40,200,000
Minimum [Member] | MYLTIP 2014 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Potential Awards Earned		0
Maximum [Member] | MYLTIP 2014 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Potential Awards Earned		40,200,000
Threshold [Member] | MYLTIP 2014 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Amount of Each Tier		6,700,000
Target [Member] | MYLTIP 2014 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Amount of Each Tier		13,400,000
High [Member] | MYLTIP 2014 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Amount of Each Tier		26,800,000
Chief Executive Officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
LTIP units issued (in shares)					24,231
MYLTIP units issued					38,926
Non-qualified stock options issued (in shares)					50,847
Board of Directors Chairman [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Cash Payment Related to Transition Agreement						6,700,000
Target Equity Award Compensation Associated with Transition Agreement						11,100,000
Compensation Expense associated with Transition Agreement			3,900,000	13,800,000
Acceleration of Stock Based Compensation in Association with Transition Agreement				$12,900,000
Cohen & Steers Realty Majors Portfolio Index [Member] | MYLTIP 2014 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted Percentage of Index Used to Compare to TRS		50.00%
NAREIT Office Index adjusted [Member] | MYLTIP 2014 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted Percentage of Index Used to Compare to TRS		50.00%
MYLTIP Vesting 2017 [Member] | MYLTIP 2014 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting Percentage		50.00%
Vesting Date		3-Feb-17
MYLTIP vesting 2018 [Member] | MYLTIP 2014 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting Percentage		50.00%
Vesting Date		3-Feb-18

Stock_Option_and_Incentive_Pla3

Stock Option and Incentive Plan (Restricted Stock) (Narrative) (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Apr. 02, 2013	Mar. 11, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares issued	153,192,845	153,062,001
Stock-based compensation associated with restricted stock, non-qualified options, LTIP Units, 2008 OPP Units, 2011 OPP Units, and 2012 OPP Units	$26,000,000	$43,900,000	$28,300,000
Value of shares of common stock	1,531,000	1,530,000
Weighted-average grant date fair value (in dollars per share)		$104.50	$107.23
Exercisable, Weighted Average Exercise Price		$98.83	$98.92
Number Exercisable at 12/31/2013 and 12/31/2012		199,868	91,496
Total intrinsic value of the outstanding and exercisable stock options	13,100,000
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares issued	23,968	36,730	20,756
Employee and director payment per share (in dollars per share)	$0.01
Value of shares of common stock	2,600,000	3,900,000	2,200,000
Employee's weighted average cost per share (in dollars per share)	$109.27	$105.30	$107.31
LTIP Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
LTIP units issued (in shares)	127,094	184,733	174,650
Value of LTIP units issued	12,800,000	17,800,000	17,300,000
Per unit fair value weighted-average (in dollars per share)	$100.61	$96.13	$98.83
Expected life assumed to calculate per unit fair value per LTIP unit (years)	5 years 8 months 12 days	5 years 8 months 12 days	5 years 9 months 19 days
Risk-free rate	1.84%	1.03%	0.94%
Expected price volatility	27.00%	26.00%	29.10%
LTIPs And Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting annual installments	4
Unrecognized compensation expenses	17,000,000
LTIPs, OPP and MYLTIP Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Employee and director payment per share (in dollars per share)	$0.25
OPP Units 2012 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
2012 OPP units issued (in shares)			400,000
MYLTIP 2013 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
MYLTIP units issued		318,926
Non Qualified Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Non-qualified stock options issued (in shares)	0	252,220	186,007
Expected life assumed to calculate per unit fair value per LTIP unit (years)		6 years 0 months 0 days	5 years 4 months 24 days
Risk-free rate		1.11%	0.92%
Expected price volatility		26.00%	28.40%
Expected dividend rate		3.00%	2.90%
Weighted-average grant date fair value (in dollars per share)		$18.46	$19.50
Weighted-average exercise price (in dollars per share)		$105.10	$107.23
Unvested 2012 OPP Units and 2013 and 2014 MYLTIP Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation expenses	14,500,000
Weighted-average period (years)	2 years 8 months 12 days
Non-Qualified Employee Stock Purchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum Common Stock available for issuance	250,000
Employee Common Stock options, percent of average closing price	85.00%
Number Of Days Used In Calculation, Average Closing Prices Of Common Stock	10
Shares issued	6,964	6,442	7,406
Shares issued, weighted-average purchase price per share	$93.37	$89.65	$86.52
Chief Executive Officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
LTIP units issued (in shares)				24,231
Non-qualified stock options issued (in shares)				50,847
MYLTIP units issued				38,926
Board of Directors Chairman [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation associated with restricted stock, non-qualified options, LTIP Units, 2008 OPP Units, 2011 OPP Units, and 2012 OPP Units	2,500,000	21,500,000
Compensation Expense associated with Transition Agreement	3,900,000	13,800,000
Acceleration of Stock Based Compensation in Association with Transition Agreement		12,900,000
Cash Payment Related to Transition Agreement					6,700,000
Target Equity Award Compensation Associated with Transition Agreement					11,100,000
Executive Vice President [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation associated with restricted stock, non-qualified options, LTIP Units, 2008 OPP Units, 2011 OPP Units, and 2012 OPP Units	$2,700,000

Stock_Option_and_Incentive_Pla4

Stock Option and Incentive Plan Schedule of Stock Options Status (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Beginning Outstanding, Shares	558,823	294,527	155,623
Beginning Oustanding, Weighted Average Exercise Price	$100.43	$101.06	$89.35
Granted, Shares		252,220	186,007
Granted, Weighted Average Exercise Price		$104.50	$107.23
Exercised, Shares	-21,459	0	-22,823
Exercised, Weighted Averge Exercise Price	$97.04		$72.42
Canceled, Shares	-2,444		-24,280
Canceled, Weighted Average Exercise Price	$103.57		$100.15
Special dividend adjustment (shares)	18,392	12,076
Special dividend adjustment weighted average exercise price	$97.22	$100.44
Ending Outstanding, Shares	553,312	558,823	294,527
Ending Oustanding, Weighted Average Exercise Pric	$97.21	$100.43	$101.06

Stock_Option_and_Incentive_Pla5

Stock Option and Incentive Plan Schedule of Stock Options Outstanding (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Number Outstanding at 12/31/14	553,312	558,823	294,527	155,623
Outstanding, Weighted Average Exercise Price	$97.21	$100.43	$101.06	$89.35
Number Exercisable at 12/31/2014		199,868	91,496
Exercisable, Weighted Average Exercise Price		$98.83	$98.92
Exercise Price of $87.70 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Weighted Average Remaining Contractual Life (Years)	3 years 7 months 6 days
Number Outstanding at 12/31/14	124,513
Outstanding, Weighted Average Exercise Price	$87.70
Number Exercisable at 12/31/2014	111,832
Exercisable, Weighted Average Exercise Price	$87.70
Exercise Price of $96.62 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Weighted Average Remaining Contractual Life (Years)	8 years 3 months 18 days
Number Outstanding at 12/31/14	53,759
Outstanding, Weighted Average Exercise Price	$96.62
Number Exercisable at 12/31/2014	13,439
Exercisable, Weighted Average Exercise Price	$96.62
Exercise Price of $99.41 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Weighted Average Remaining Contractual Life (Years)	4 years 10 months 24 days
Number Outstanding at 12/31/14	207,064
Outstanding, Weighted Average Exercise Price	$99.41
Number Exercisable at 12/31/2014	150,024
Exercisable, Weighted Average Exercise Price	$99.41
Exercise Price of $101.75 Cents [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Weighted Average Remaining Contractual Life (Years)	4 years 4 months 24 days
Number Outstanding at 12/31/14	167,976
Outstanding, Weighted Average Exercise Price	$101.75
Number Exercisable at 12/31/2014	135,848
Exercisable, Weighted Average Exercise Price	$101.75

Related_Party_Transactions_Det

Related Party Transactions (Details) (USD $)	12 Months Ended			0 Months Ended	12 Months Ended	76 Months Ended	122 Months Ended	198 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Jun. 30, 1998	Dec. 31, 2004	Oct. 20, 2004	Dec. 31, 2014	Dec. 31, 2014	20-May-14	Nov. 12, 2014
	nonemployeedirectors							sqft
								Y
Related Party Transaction [Line Items]
Number Of Non employee Directors Electing To Receive Deferred Stock Units	6
Deferred Stock Units issued									7,542
Deferred Stock Units Outstanding	84,435	83,995					84,435	84,435
New York regional manager [Member]
Related Party Transaction [Line Items]
Aggregate leasing commissions paid to related parties	1,214,000	$592,000
Brother of Executive Vice President [Member]
Related Party Transaction [Line Items]
Aggregate leasing commissions paid to related parties	674,000	868,000	1,306,000
Former Director [Member]
Related Party Transaction [Line Items]
Related Party Acquisition Date				30-Jun-98
Annual Development Agreement Payments.	125,000
Development Agreement Payment Made For 1998 Through 2004					750,000
Affiliate of Former Director [Member]
Related Party Transaction [Line Items]
Square Feet of Development								2,000,000
Purchase price per rentable square feet of property developed.								20
Target Annual Return Minimum						10.50%
Target Annual Return Maximum.						11.00%
Aggregate Fixed Rentable Per Square Foot Of Developed Property.							10.5
Adjustment To Aggregate Fixed Amount Per Square Foot Of Developed Property							5.5
Term of Development Agreement., Years								20
804 carnegie center [Member]
Related Party Transaction [Line Items]
Aggregate purchase price										$3,700,000

Selected_Interim_Financial_Inf2

Selected Interim Financial Information (unaudited) (Details) (USD $)	3 Months Ended								12 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Quarterly Financial Information Disclosure [Abstract]
Total revenue	$613,707	$618,803	$589,794	$574,694	$576,199	$571,481	$510,033	$477,826	$2,396,998	$2,135,539	$1,847,186
Income from continuing operations	85,323	109,038	95,901	67,756	79,395	82,311	503,446	38,496	358,018	703,648	285,456
Net income attributable to Boston Properties, Inc. common sharedholders	$174,510	$127,724	$76,527	$54,034	$88,719	$152,677	$452,417	$47,854	$433,111	$741,754	$289,650
Income attributable to Boston Properties, Inc. per share—basic	$1.14	$0.83	$0.50	$0.35	$0.58	$1	$2.95	$0.32	$2.83	$4.87	$1.93
Income attributable to Boston Properties, Inc. per share—diluted	$1.14	$0.83	$0.50	$0.35	$0.58	$1	$2.94	$0.31	$2.83	$4.86	$1.92

Subsequent_Events_subsequent_E

Subsequent Events subsequent Events (Details) (USD $)	0 Months Ended
	Jan. 21, 2015	Feb. 06, 2015	Jan. 15, 2015	Dec. 31, 2014	Dec. 31, 2013	Feb. 19, 2015	Feb. 03, 2015
	tiers
	indices
	Y
Subsequent Event [Line Items]
Common stock, shares issued				153,192,845	153,062,001
Subsequent Event [Member]
Subsequent Event [Line Items]
LTIP units issued (in shares)							96,830
Derivative, Average Fixed Interest Rate						2.51%
Derivative, Notional Amount						$125,000,000
Subsequent Event [Member] | MYLTIP 2015 [Member]
Subsequent Event [Line Items]
Indices Used to Compare TRS	2
Number of Tiers	3
Percentage of annualized TRS for Reduction of Earned Awards	0.00%
Percentage to Cause Some Awards to be Earned Even if on a Relative Basis it Would Not Result in any Earned Awards	12.00%
Distributions percent before measurement date	10.00%
Value of MYLTIP Awards	15,700,000
MYLTIP Value Amortized Into Earnings, Years	4
TRS measurement (in years)	3
Subsequent Event [Member] | MYLTIP 2015 [Member] | Cohen & Steers Realty Majors Portfolio Index [Member]
Subsequent Event [Line Items]
Weighted Percentage of Index Used to Compare to TRS	50.00%
Subsequent Event [Member] | MYLTIP 2015 [Member] | NAREIT Office Index adjusted [Member]
Subsequent Event [Line Items]
Weighted Percentage of Index Used to Compare to TRS	50.00%
Subsequent Event [Member] | OPP Units 2012 [Member]
Subsequent Event [Line Items]
Outperformance awards earned		32,100,000
potential maximum amount of Outperformance Awards earned (percent)		0.8
Subsequent Event [Member] | High [Member] | MYLTIP 2015 [Member]
Subsequent Event [Line Items]
Amount of Each Tier	40,800,000
Subsequent Event [Member] | Minimum [Member] | MYLTIP 2015 [Member]
Subsequent Event [Line Items]
Potential Awards Earned	0
Subsequent Event [Member] | Maximum [Member] | MYLTIP 2015 [Member]
Subsequent Event [Line Items]
Potential Awards Earned	40,800,000
Subsequent Event [Member] | Maximum [Member] | OPP Units 2012 [Member]
Subsequent Event [Line Items]
Potential Awards Earned		40,000,000
Subsequent Event [Member] | Threshold [Member] | MYLTIP 2015 [Member]
Subsequent Event [Line Items]
Amount of Each Tier	8,200,000
Subsequent Event [Member] | Target [Member] | MYLTIP 2015 [Member]
Subsequent Event [Line Items]
Amount of Each Tier	16,300,000
Subsequent Event [Member] | MYLTIP Vesting 2018 [Member] | MYLTIP 2015 [Member]
Subsequent Event [Line Items]
Vesting Percentage	50.00%
Vesting Date	4-Feb-18
Subsequent Event [Member] | MYLTIP vesting 2019 [Member] | MYLTIP 2015 [Member]
Subsequent Event [Line Items]
Vesting Percentage	50.00%
Vesting Date	4-Feb-19
Subsequent Event [Member] | Restricted Stock [Member]
Subsequent Event [Line Items]
Common stock, shares issued							30,965
Residences on The Avenue, Two Two Two One I St., NW [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Sale Price Of Sold Property			196,000,000
Rental revenue supoprt payments to buyer maximum			6,000,000
Number of apartment units			335
Area of Real Estate Property			50,000
Washingtonian North [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Sale Price Of Sold Property						$8,700,000
Are of land sold						8.5
Area of Land						27

Real_Estate_and_Accumulated_De2

Real Estate and Accumulated Depreciation Schedule of Real Estate and Accumulated Depreciation (Details) (USD $)	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Real Estate and Accumulated Depreciation [Line Items]
Encumbrances	$4,309,484,000	[1]
Original Land	4,661,817,000
Original Building	11,331,324,000
Costs Capitalized Subsequent to Acquisition	3,215,276,000
Land and improvements	4,785,772,000
Buildings and Improvements	13,418,220,000
Land Held for Development	268,114,000
Development and Construction in Progress	736,311,000
Total	19,208,417,000		18,953,601,000	14,869,887,000	13,363,113,000
Accumulated Depreciation	3,529,978,000		3,145,701,000	2,919,479,000	2,626,324,000
Furniture, fixtures and equipment	27,986,000		25,164,000
Furniture, fixtures and equipment accumulated depreciation	17,681,000
Real Estate Aggregate Cost For Tax Purposes	14,300,000,000
Real Estate Aggregate Accumulated Depreciation For Tax Purposes	2,600,000,000
Real Estate and Accumulated Depreciation, Life Used for Depreciation	40 years 0 months 0 days
Unamortized balance of historical fair value adjustments	138,711,000
767 5th Avenue (The General Motors Building) [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	767 Fifth Avenue (The General Motors Building)
Type	Office
Location	New York, NY
Encumbrances	1,421,083,000
Original Land	1,796,252,000
Original Building	1,532,654,000
Costs Capitalized Subsequent to Acquisition	19,627,000
Land and improvements	1,796,252,000
Buildings and Improvements	1,552,281,000
Land Held for Development	0
Development and Construction in Progress	0
Total	3,348,533,000
Accumulated Depreciation	80,075,000
Year(s) built / renovated	1968
Depreciable Lives (Years)	-1	[2]
Embarcadero Center [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	Embarcadero Center
Type	Office
Location	San Francisco, CA
Encumbrances	354,680,000
Original Land	179,697,000
Original Building	847,410,000
Costs Capitalized Subsequent to Acquisition	317,320,000
Land and improvements	195,987,000
Buildings and Improvements	1,148,440,000
Land Held for Development	0
Development and Construction in Progress	0
Total	1,344,427,000
Accumulated Depreciation	505,702,000
Year(s) built / renovated	1970/1989
Depreciable Lives (Years)	-1	[2]
Prudential Center [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	Prudential Center
Type	Office
Location	Boston, MA
Original Land	92,077,000
Original Building	734,594,000
Costs Capitalized Subsequent to Acquisition	380,216,000
Land and improvements	107,426,000
Buildings and Improvements	1,064,013,000
Land Held for Development	404,000
Development and Construction in Progress	35,044,000
Total	1,206,887,000
Accumulated Depreciation	421,371,000
Year(s) built / renovated	1965/1993/2002
Depreciable Lives (Years)	-1	[2]
399 Park Avenue [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	399 Park Avenue
Type	Office
Location	New York, NY
Original Land	339,200,000
Original Building	700,358,000
Costs Capitalized Subsequent to Acquisition	109,630,000
Land and improvements	354,107,000
Buildings and Improvements	795,081,000
Land Held for Development	0
Development and Construction in Progress	0
Total	1,149,188,000
Accumulated Depreciation	242,572,000
Year(s) built / renovated	1961
Depreciable Lives (Years)	-1	[2]
601 Lexington Avenue [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	601 Lexington Avenue
Type	Office
Location	New York, NY
Encumbrances	710,932,000
Original Land	241,600,000
Original Building	494,782,000
Costs Capitalized Subsequent to Acquisition	230,137,000
Land and improvements	289,639,000
Buildings and Improvements	676,880,000
Land Held for Development	0
Development and Construction in Progress	0
Total	966,519,000
Accumulated Depreciation	228,397,000
Year(s) built / renovated	1977/1997
Depreciable Lives (Years)	-1	[2]
The John Hancock Tower And Garage [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	The John Hancock Tower and Garage
Type	Office
Location	Boston, MA
Encumbrances	649,108,000
Original Land	219,543,000
Original Building	667,884,000
Costs Capitalized Subsequent to Acquisition	74,060,000
Land and improvements	219,616,000
Buildings and Improvements	740,863,000
Land Held for Development	1,008,000
Development and Construction in Progress	0
Total	961,487,000
Accumulated Depreciation	104,888,000
Year(s) built / renovated	1976
Depreciable Lives (Years)	-1	[2]
250 West 55th Street [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	250 West 55th Street
Type	Office
Location	New York, NY
Encumbrances	0
Original Land	285,263,000
Original Building	603,167,000
Costs Capitalized Subsequent to Acquisition	0
Land and improvements	285,263,000
Buildings and Improvements	603,167,000
Land Held for Development	0
Development and Construction in Progress	0
Total	888,430,000
Accumulated Depreciation	9,240,000
Year(s) built / renovated	2014
Depreciable Lives (Years)	-1	[2]
Times Square Tower [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	Times Square Tower
Type	Office
Location	New York, NY
Original Land	165,413,000
Original Building	380,438,000
Costs Capitalized Subsequent to Acquisition	84,488,000
Land and improvements	169,193,000
Buildings and Improvements	461,146,000
Land Held for Development	0
Development and Construction in Progress	0
Total	630,339,000
Accumulated Depreciation	150,619,000
Year(s) built / renovated	2004
Depreciable Lives (Years)	-1	[2]
100 Federal Street [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	100 Federal Street
Type	Office
Location	Boston, MA
Original Land	131,067,000
Original Building	435,954,000
Costs Capitalized Subsequent to Acquisition	9,711,000
Land and improvements	131,067,000
Buildings and Improvements	445,432,000
Land Held for Development	233,000
Development and Construction in Progress	0
Total	576,732,000
Accumulated Depreciation	44,931,000
Year(s) built / renovated	1971-1975
Depreciable Lives (Years)	-1	[2]
Carnegie Center [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	Carnegie Center
Type	Office
Location	Princeton, NJ
Original Land	105,107,000
Original Building	377,259,000
Costs Capitalized Subsequent to Acquisition	83,053,000
Land and improvements	103,064,000
Buildings and Improvements	450,653,000
Land Held for Development	2,350,000
Development and Construction in Progress	9,352,000
Total	565,419,000
Accumulated Depreciation	183,866,000
Year(s) built / renovated	1983-1999
Depreciable Lives (Years)	-1	[2]
Atlantic Wharf [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	Atlantic Wharf
Type	Office
Location	Boston, MA
Original Land	63,988,000
Original Building	454,537,000
Costs Capitalized Subsequent to Acquisition	15,169,000
Land and improvements	63,988,000
Buildings and Improvements	469,706,000
Land Held for Development	0
Development and Construction in Progress	0
Total	533,694,000
Accumulated Depreciation	55,070,000
Year(s) built / renovated	2011
Depreciable Lives (Years)	-1	[2]
Fountain Square [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	Fountain Square
Type	Office
Location	Reston, VA
Encumbrances	220,133,000
Original Land	56,853,000
Original Building	306,298,000
Costs Capitalized Subsequent to Acquisition	6,938,000
Land and improvements	56,853,000
Buildings and Improvements	313,236,000
Land Held for Development	0
Development and Construction in Progress	0
Total	370,089,000
Accumulated Depreciation	27,645,000
Year(s) built / renovated	1986-1990
Depreciable Lives (Years)	-1	[2]
510 Madison Avenue [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	510 Madison Avenue
Type	Office
Location	New York, NY
Original Land	103,000,000
Original Building	253,665,000
Costs Capitalized Subsequent to Acquisition	12,912,000
Land and improvements	103,000,000
Buildings and Improvements	266,577,000
Land Held for Development	0
Development and Construction in Progress	0
Total	369,577,000
Accumulated Depreciation	24,871,000
Year(s) built / renovated	2012
Depreciable Lives (Years)	-1	[2]
599 Lexington Avenue [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	599 Lexington Avenue
Type	Office
Location	New York, NY
Encumbrances	750,000,000
Original Land	81,040,000
Original Building	100,507,000
Costs Capitalized Subsequent to Acquisition	128,649,000
Land and improvements	87,852,000
Buildings and Improvements	222,344,000
Land Held for Development	0
Development and Construction in Progress	0
Total	310,196,000
Accumulated Depreciation	152,958,000
Year(s) built / renovated	1986
Depreciable Lives (Years)	-1	[2]
680 Folsom Street [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	680 Folsom Street
Type	Office
Location	San Francisco, CA
Encumbrances	0
Original Land	106,510,000
Original Building	185,801,000
Costs Capitalized Subsequent to Acquisition	0
Land and improvements	106,510,000
Buildings and Improvements	185,801,000
Land Held for Development	0
Development and Construction in Progress	0
Total	292,311,000
Accumulated Depreciation	4,161,000
Year(s) built / renovated	2014
Depreciable Lives (Years)	-1	[2]
South of Market and Democracy Tower [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	South of Market and Democracy Tower
Type	Office
Location	Reston, VA
Original Land	13,603,000
Original Building	237,479,000
Costs Capitalized Subsequent to Acquisition	9,790,000
Land and improvements	13,687,000
Buildings and Improvements	247,185,000
Development and Construction in Progress	0
Total	260,872,000
Accumulated Depreciation	60,668,000
Year(s) built / renovated	2008-2009
Depreciable Lives (Years)	-1	[2]
Bay Colony Corporate Center [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	Bay Colony Corporate Center
Type	Office
Location	Waltham, MA
Original Land	18,789,000
Original Building	148,451,000
Costs Capitalized Subsequent to Acquisition	58,126,000
Land and improvements	18,789,000
Buildings and Improvements	206,577,000
Land Held for Development	0
Development and Construction in Progress	0
Total	225,366,000
Accumulated Depreciation	31,089,000
Year(s) built / renovated	1985-1989
Depreciable Lives (Years)	-1	[2]
Gateway Center [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	Gateway Center
Type	Office
Location	San Francisco, CA
Original Land	28,255,000
Original Building	139,245,000
Costs Capitalized Subsequent to Acquisition	52,768,000
Land and improvements	30,627,000
Buildings and Improvements	189,641,000
Land Held for Development	0
Development and Construction in Progress	0
Total	220,268,000
Accumulated Depreciation	87,759,000
Year(s) built / renovated	1984/1986/2002
Depreciable Lives (Years)	-1	[2]
2200 Pennsylvania Avenue [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	2200 Pennsylvania Avenue
Type	Office
Location	Washington, DC
Original Building	183,541,000
Costs Capitalized Subsequent to Acquisition	5,141,000
Land and improvements	0
Buildings and Improvements	188,682,000
Land Held for Development	0
Development and Construction in Progress	0
Total	188,682,000
Accumulated Depreciation	26,612,000
Year(s) built / renovated	2011
Depreciable Lives (Years)	-1	[2]
Mountain View Research [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	Mountain View Research Park
Type	Office
Location	Mountain View, CA
Original Land	95,066,000
Original Building	68,373,000
Costs Capitalized Subsequent to Acquisition	3,267,000
Land and improvements	95,066,000
Buildings and Improvements	71,640,000
Land Held for Development	0
Development and Construction in Progress	0
Total	166,706,000
Accumulated Depreciation	6,044,000
Year(s) built / renovated	1977-1981/2007-2013
Depreciable Lives (Years)	-1	[2]
3100-3130 Zanker Road (formerly 3200 Zanker Road) [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	3100-3130 Zanker Road (formerly 3200 Zanker Road)
Type	Office
Location	San Jose, CA
Original Land	36,705,000
Original Building	82,863,000
Costs Capitalized Subsequent to Acquisition	28,919,000
Land and improvements	36,997,000
Buildings and Improvements	107,737,000
Land Held for Development	3,753,000
Development and Construction in Progress	0
Total	148,487,000
Accumulated Depreciation	25,914,000
Year(s) built / renovated	1988
Depreciable Lives (Years)	-1	[2]
Reservoir Place [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	Reservoir Place
Type	Office
Location	Waltham, MA
Original Land	18,605,000
Original Building	92,619,000
Costs Capitalized Subsequent to Acquisition	34,732,000
Land and improvements	20,118,000
Buildings and Improvements	125,830,000
Land Held for Development	8,000
Development and Construction in Progress	0
Total	145,956,000
Accumulated Depreciation	57,456,000
Year(s) built / renovated	1955/1987
Depreciable Lives (Years)	-1	[2]
1333 New Hampshire Avenue [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	1333 New Hampshire Avenue
Type	Office
Location	Washington, DC
Original Land	34,032,000
Original Building	85,660,000
Costs Capitalized Subsequent to Acquisition	9,947,000
Land and improvements	35,382,000
Buildings and Improvements	94,257,000
Land Held for Development	0
Development and Construction in Progress	0
Total	129,639,000
Accumulated Depreciation	32,942,000
Year(s) built / renovated	1996
Depreciable Lives (Years)	-1	[2]
505 9th Street [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	505 9th Street
Type	Office
Location	Washington, DC
Encumbrances	118,919,000
Original Land	38,885,000
Original Building	83,719,000
Costs Capitalized Subsequent to Acquisition	5,692,000
Land and improvements	42,082,000
Buildings and Improvements	86,214,000
Land Held for Development	0
Development and Construction in Progress	0
Total	128,296,000
Accumulated Depreciation	23,366,000
Year(s) built / renovated	2007
Depreciable Lives (Years)	-1	[2]
Kingstowne Towne Center [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	Kingstowne Towne Center
Type	Office
Location	Alexandria, VA
Encumbrances	31,364,000
Original Land	18,021,000
Original Building	109,038,000
Costs Capitalized Subsequent to Acquisition	1,013,000
Land and improvements	18,062,000
Buildings and Improvements	110,010,000
Land Held for Development	0
Development and Construction in Progress	0
Total	128,072,000
Accumulated Depreciation	29,821,000
Year(s) built / renovated	2003-2006
Depreciable Lives (Years)	-1	[2]
1330 Connecticut Avenue [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	1330 Connecticut Avenue
Type	Office
Location	Washington, DC
Original Land	25,982,000
Original Building	82,311,000
Costs Capitalized Subsequent to Acquisition	16,379,000
Land and improvements	27,135,000
Buildings and Improvements	97,537,000
Land Held for Development	0
Development and Construction in Progress	0
Total	124,672,000
Accumulated Depreciation	32,742,000
Year(s) built / renovated	1984
Depreciable Lives (Years)	-1	[2]
Capital Gallery [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	Capital Gallery
Type	Office
Location	Washington, DC
Original Land	4,725,000
Original Building	29,565,000
Costs Capitalized Subsequent to Acquisition	88,078,000
Land and improvements	8,662,000
Buildings and Improvements	113,706,000
Land Held for Development	0
Development and Construction in Progress	0
Total	122,368,000
Accumulated Depreciation	53,320,000
Year(s) built / renovated	1981/2006
Depreciable Lives (Years)	-1	[2]
Weston Corporate Center [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	Weston Corporate Center
Type	Office
Location	Weston, MA
Original Land	25,753,000
Original Building	92,312,000
Costs Capitalized Subsequent to Acquisition	-123,000
Land and improvements	25,854,000
Buildings and Improvements	92,088,000
Land Held for Development	0
Development and Construction in Progress	0
Total	117,942,000
Accumulated Depreciation	13,972,000
Year(s) built / renovated	2010
Depreciable Lives (Years)	-1	[2]
One Freedom Square [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	One Freedom Square
Type	Office
Location	Reston, VA
Encumbrances	0
Original Land	9,929,000
Original Building	84,504,000
Costs Capitalized Subsequent to Acquisition	18,898,000
Land and improvements	11,293,000
Buildings and Improvements	102,038,000
Land Held for Development	0
Development and Construction in Progress	0
Total	113,331,000
Accumulated Depreciation	36,912,000
Year(s) built / renovated	2000
Depreciable Lives (Years)	-1	[2]
Two Freedom Square [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	Two Freedom Square
Type	Office
Location	Reston, VA
Encumbrances	0
Original Land	13,930,000
Original Building	77,739,000
Costs Capitalized Subsequent to Acquisition	19,563,000
Land and improvements	15,420,000
Buildings and Improvements	95,812,000
Land Held for Development	0
Development and Construction in Progress	0
Total	111,232,000
Accumulated Depreciation	38,275,000
Year(s) built / renovated	2001
Depreciable Lives (Years)	-1	[2]
One And Two Reston Overlook [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	One and Two Reston Overlook
Type	Office
Location	Reston, VA
Encumbrances	0
Original Land	16,456,000
Original Building	66,192,000
Costs Capitalized Subsequent to Acquisition	25,320,000
Land and improvements	16,179,000
Buildings and Improvements	91,789,000
Land Held for Development	0
Development and Construction in Progress	0
Total	107,968,000
Accumulated Depreciation	35,274,000
Year(s) built / renovated	1999
Depreciable Lives (Years)	-1	[2]
415 Main Street (formerly Seven Cambridge Center) [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	415 Main Street (formerly Seven Cambridge Center)
Type	Office
Location	Cambridge, MA
Encumbrances	0
Original Land	3,457,000
Original Building	97,136,000
Costs Capitalized Subsequent to Acquisition	2,880,000
Land and improvements	4,125,000
Buildings and Improvements	99,348,000
Land Held for Development	0
Development and Construction in Progress	0
Total	103,473,000
Accumulated Depreciation	53,469,000
Year(s) built / renovated	2006
Depreciable Lives (Years)	-1	[2]
Discovery Square [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	Discovery Square
Type	Office
Location	Reston, VA
Encumbrances	0
Original Land	11,198,000
Original Building	71,782,000
Costs Capitalized Subsequent to Acquisition	19,977,000
Land and improvements	12,533,000
Buildings and Improvements	90,424,000
Land Held for Development	0
Development and Construction in Progress	0
Total	102,957,000
Accumulated Depreciation	32,326,000
Year(s) built / renovated	2001
Depreciable Lives (Years)	-1	[2]
140 Kendrick Street [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	140 Kendrick Street
Type	Office
Location	Needham, MA
Encumbrances	0
Original Land	18,095,000
Original Building	66,905,000
Costs Capitalized Subsequent to Acquisition	14,870,000
Land and improvements	19,092,000
Buildings and Improvements	80,778,000
Land Held for Development	0
Development and Construction in Progress	0
Total	99,870,000
Accumulated Depreciation	20,193,000
Year(s) built / renovated	2000
Depreciable Lives (Years)	-1	[2]
355 Main Street (formerly Five Cambridge Center) [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	355 Main Street (formerly Five Cambridge Center)
Type	Office
Location	Cambridge, MA
Encumbrances	0
Original Land	18,863,000
Original Building	53,346,000
Costs Capitalized Subsequent to Acquisition	25,899,000
Land and improvements	21,173,000
Buildings and Improvements	76,935,000
Land Held for Development	0
Development and Construction in Progress	0
Total	98,108,000
Accumulated Depreciation	20,864,000
Year(s) built / renovated	1981/1996/2013
Depreciable Lives (Years)	-1	[2]
90 Broadway (formerly Four Cambridge Center) [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	90 Broadway (formerly Four Cambridge Center)
Type	Office
Location	Cambridge, MA
Encumbrances	0
Original Land	19,104,000
Original Building	52,078,000
Costs Capitalized Subsequent to Acquisition	13,680,000
Land and improvements	20,785,000
Buildings and Improvements	64,077,000
Land Held for Development	0
Development and Construction in Progress	0
Total	84,862,000
Accumulated Depreciation	12,426,000
Year(s) built / renovated	1983/1998/2013
Depreciable Lives (Years)	-1	[2]
Waltham Weston Corporate Center [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	Waltham Weston Corporate Center
Type	Office
Location	Waltham, MA
Encumbrances	0
Original Land	10,385,000
Original Building	60,694,000
Costs Capitalized Subsequent to Acquisition	8,105,000
Land and improvements	11,097,000
Buildings and Improvements	68,087,000
Land Held for Development	0
Development and Construction in Progress	0
Total	79,184,000
Accumulated Depreciation	22,151,000
Year(s) built / renovated	2003
Depreciable Lives (Years)	-1	[2]
230 CityPoint [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	230 CityPoint
Type	Office
Location	Waltham, MA
Encumbrances	0
Original Land	13,189,000
Original Building	49,823,000
Costs Capitalized Subsequent to Acquisition	15,744,000
Land and improvements	13,593,000
Buildings and Improvements	65,163,000
Land Held for Development	0
Development and Construction in Progress	0
Total	78,756,000
Accumulated Depreciation	19,029,000
Year(s) built / renovated	1992
Depreciable Lives (Years)	-1	[2]
77 CityPoint [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	77 CityPoint
Type	Office
Location	Waltham, MA
Encumbrances	0
Original Land	13,847,000
Original Building	60,383,000
Costs Capitalized Subsequent to Acquisition	3,163,000
Land and improvements	13,873,000
Buildings and Improvements	63,520,000
Land Held for Development	0
Development and Construction in Progress	0
Total	77,393,000
Accumulated Depreciation	14,639,000
Year(s) built / renovated	2008
Depreciable Lives (Years)	-1	[2]
North First Business Park [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	North First Business Park
Type	Office
Location	San Jose, CA
Encumbrances	0
Original Land	58,402,000
Original Building	13,069,000
Costs Capitalized Subsequent to Acquisition	4,028,000
Land and improvements	23,377,000
Buildings and Improvements	16,214,000
Land Held for Development	35,908,000
Development and Construction in Progress	0
Total	75,499,000
Accumulated Depreciation	12,174,000
Year(s) built / renovated	1981
Depreciable Lives (Years)	-1	[2]
300 Binney Street (formerly Seventeen Cambridge Center) [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	300 Binney Street (formerly Seventeen Cambridge Center)
Type	Office
Location	Cambridge, MA
Encumbrances	0
Original Land	18,080,000
Original Building	51,262,000
Costs Capitalized Subsequent to Acquisition	165,000
Land and improvements	18,080,000
Buildings and Improvements	51,427,000
Land Held for Development	0
Development and Construction in Progress	0
Total	69,507,000
Accumulated Depreciation	2,938,000
Year(s) built / renovated	2013
Depreciable Lives (Years)	-1	[2]
2440 West El Camino Real [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	2440 West El Camino Real
Type	Office
Location	Mountain View, CA
Encumbrances	0
Original Land	16,741,000
Original Building	51,285,000
Costs Capitalized Subsequent to Acquisition	716,000
Land and improvements	16,741,000
Buildings and Improvements	52,001,000
Land Held for Development	0
Development and Construction in Progress	0
Total	68,742,000
Accumulated Depreciation	6,400,000
Year(s) built / renovated	1987/2003
Depreciable Lives (Years)	-1	[2]
Wisconsin Place [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	Wisconsin Place
Type	Office
Location	Chevy Chase, MD
Encumbrances	0
Original Land	0
Original Building	53,349,000
Costs Capitalized Subsequent to Acquisition	12,787,000
Land and improvements	0
Buildings and Improvements	66,136,000
Land Held for Development	0
Development and Construction in Progress	0
Total	66,136,000
Accumulated Depreciation	14,300,000
Year(s) built / renovated	2009
Depreciable Lives (Years)	-1	[2]
Reston Corporate Center [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	Reston Corporate Center
Type	Office
Location	Reston, VA
Encumbrances	0
Original Land	9,135,000
Original Building	50,857,000
Costs Capitalized Subsequent to Acquisition	5,361,000
Land and improvements	10,148,000
Buildings and Improvements	55,205,000
Land Held for Development	0
Development and Construction in Progress	0
Total	65,353,000
Accumulated Depreciation	22,540,000
Year(s) built / renovated	1984
Depreciable Lives (Years)	-1	[2]
New Dominion Technology Park, Bldg. Two [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	New Dominion Technology Park, Bldg. Two
Type	Office
Location	Herndon, VA
Encumbrances	0
Original Land	5,584,000
Original Building	51,868,000
Costs Capitalized Subsequent to Acquisition	3,911,000
Land and improvements	6,510,000
Buildings and Improvements	54,853,000
Land Held for Development	0
Development and Construction in Progress	0
Total	61,363,000
Accumulated Depreciation	18,312,000
Year(s) built / renovated	2004
Depreciable Lives (Years)	-1	[2]
Sumner Square [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	Sumner Square
Type	Office
Location	Washington, DC
Encumbrances	0
Original Land	624,000
Original Building	28,745,000
Costs Capitalized Subsequent to Acquisition	25,004,000
Land and improvements	1,478,000
Buildings and Improvements	52,895,000
Land Held for Development	0
Development and Construction in Progress	0
Total	54,373,000
Accumulated Depreciation	25,152,000
Year(s) built / renovated	1985
Depreciable Lives (Years)	-1	[2]
New Dominion Technology Park, Bldg. One [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	New Dominion Technology Park, Bldg. One
Type	Office
Location	Herndon, VA
Encumbrances	40,975,000
Original Land	3,880,000
Original Building	43,227,000
Costs Capitalized Subsequent to Acquisition	3,883,000
Land and improvements	4,583,000
Buildings and Improvements	46,407,000
Land Held for Development	0
Development and Construction in Progress	0
Total	50,990,000
Accumulated Depreciation	21,158,000
Year(s) built / renovated	2001
Depreciable Lives (Years)	-1	[2]
200 West Street [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	200 West Street
Type	Office
Location	Waltham, MA
Encumbrances	0
Original Land	16,148,000
Original Building	24,983,000
Costs Capitalized Subsequent to Acquisition	9,782,000
Land and improvements	16,813,000
Buildings and Improvements	34,100,000
Land Held for Development	0
Development and Construction in Progress	0
Total	50,913,000
Accumulated Depreciation	16,439,000
Year(s) built / renovated	1999
Depreciable Lives (Years)	-1	[2]
191 Spring Street [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	191 Spring Street
Type	Office
Location	Lexington, MA
Encumbrances	0
Original Land	2,850,000
Original Building	27,166,000
Costs Capitalized Subsequent to Acquisition	18,558,000
Land and improvements	3,151,000
Buildings and Improvements	45,423,000
Land Held for Development	0
Development and Construction in Progress	0
Total	48,574,000
Accumulated Depreciation	31,087,000
Year(s) built / renovated	1971/1995
Depreciable Lives (Years)	-1	[2]
255 Main Street (One Cambridge Center) [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	255 Main Street (formerly One Cambridge Center)
Type	Office
Location	Cambridge, MA
Encumbrances	0
Original Land	134,000
Original Building	25,110,000
Costs Capitalized Subsequent to Acquisition	20,596,000
Land and improvements	548,000
Buildings and Improvements	45,292,000
Land Held for Development	0
Development and Construction in Progress	0
Total	45,840,000
Accumulated Depreciation	27,168,000
Year(s) built / renovated	1987
Depreciable Lives (Years)	-1	[2]
University Place [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	University Place
Type	Office
Location	Cambridge, MA
Encumbrances	12,290,000
Original Land	0
Original Building	37,091,000
Costs Capitalized Subsequent to Acquisition	6,794,000
Land and improvements	390,000
Buildings and Improvements	43,495,000
Land Held for Development	0
Development and Construction in Progress	0
Total	43,885,000
Accumulated Depreciation	22,156,000
Year(s) built / renovated	1985
Depreciable Lives (Years)	-1	[2]
2600 Tower Oaks Boulevard [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	2600 Tower Oaks Boulevard
Type	Office
Location	Rockville, MD
Encumbrances	0
Original Land	4,243,000
Original Building	31,125,000
Costs Capitalized Subsequent to Acquisition	7,924,000
Land and improvements	4,785,000
Buildings and Improvements	38,507,000
Land Held for Development	0
Development and Construction in Progress	0
Total	43,292,000
Accumulated Depreciation	16,993,000
Year(s) built / renovated	2001
Depreciable Lives (Years)	-1	[2]
Quorum Office Park [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	Quorum Office Park
Type	Office
Location	Chelmsford, MA
Encumbrances	0
Original Land	3,750,000
Original Building	32,454,000
Costs Capitalized Subsequent to Acquisition	5,814,000
Land and improvements	5,187,000
Buildings and Improvements	36,831,000
Land Held for Development	0
Development and Construction in Progress	0
Total	42,018,000
Accumulated Depreciation	13,710,000
Year(s) built / renovated	2001
Depreciable Lives (Years)	-1	[2]
500 E Street [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	500 E Street
Type	Office
Location	Washington, DC
Encumbrances	0
Original Land	109,000
Original Building	22,420,000
Costs Capitalized Subsequent to Acquisition	13,020,000
Land and improvements	2,379,000
Buildings and Improvements	33,170,000
Land Held for Development	0
Development and Construction in Progress	0
Total	35,549,000
Accumulated Depreciation	21,601,000
Year(s) built / renovated	1987
Depreciable Lives (Years)	-1	[2]
150 Broadway (formerly Eight Cambridge Center) [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	150 Broadway (formerly Eight Cambridge Center)
Type	Office
Location	Cambridge, MA
Encumbrances	0
Original Land	850,000
Original Building	25,042,000
Costs Capitalized Subsequent to Acquisition	8,507,000
Land and improvements	1,323,000
Buildings and Improvements	33,076,000
Land Held for Development	0
Development and Construction in Progress	0
Total	34,399,000
Accumulated Depreciation	12,606,000
Year(s) built / renovated	1999
Depreciable Lives (Years)	-1	[2]
325 Main Street (formerly Three Cambridge Center) [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	325 Main Street (formerly Three Cambridge Center)
Type	Office
Location	Cambridge, MA
Encumbrances	0
Original Land	174,000
Original Building	12,200,000
Costs Capitalized Subsequent to Acquisition	11,774,000
Land and improvements	965,000
Buildings and Improvements	23,183,000
Land Held for Development	0
Development and Construction in Progress	0
Total	24,148,000
Accumulated Depreciation	10,496,000
Year(s) built / renovated	1987/2013
Depreciable Lives (Years)	-1	[2]
105 Broadway (formerly Ten Cambridge Center) [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	105 Broadway (formerly Ten Cambridge Center)
Type	Office
Location	Cambridge, MA
Encumbrances	0
Original Land	1,299,000
Original Building	12,943,000
Costs Capitalized Subsequent to Acquisition	6,851,000
Land and improvements	2,395,000
Buildings and Improvements	18,698,000
Land Held for Development	0
Development and Construction in Progress	0
Total	21,093,000
Accumulated Depreciation	11,948,000
Year(s) built / renovated	1990
Depreciable Lives (Years)	-1	[2]
40 Shattuck Road [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	40 Shattuck Road
Type	Office
Location	Andover, MA
Encumbrances	0
Original Land	709,000
Original Building	14,740,000
Costs Capitalized Subsequent to Acquisition	2,697,000
Land and improvements	893,000
Buildings and Improvements	17,253,000
Land Held for Development	0
Development and Construction in Progress	0
Total	18,146,000
Accumulated Depreciation	6,171,000
Year(s) built / renovated	2001
Depreciable Lives (Years)	-1	[2]
201 Spring Street [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	201 Spring Street
Type	Office
Location	Lexington, MA
Encumbrances	0
Original Land	2,849,000
Original Building	15,303,000
Costs Capitalized Subsequent to Acquisition	-154,000
Land and improvements	3,124,000
Buildings and Improvements	14,874,000
Land Held for Development	0
Development and Construction in Progress	0
Total	17,998,000
Accumulated Depreciation	6,511,000
Year(s) built / renovated	1997
Depreciable Lives (Years)	-1	[2]
Lexington Office Park [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	Lexington Office Park
Type	Office
Location	Lexington, MA
Encumbrances	0
Original Land	998,000
Original Building	1,426,000
Costs Capitalized Subsequent to Acquisition	13,941,000
Land and improvements	1,264,000
Buildings and Improvements	15,101,000
Land Held for Development	0
Development and Construction in Progress	0
Total	16,365,000
Accumulated Depreciation	11,346,000
Year(s) built / renovated	1982
Depreciable Lives (Years)	-1	[2]
92 - 100 Hayden Avenue [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	92-100 Hayden Avenue
Type	Office
Location	Lexington, MA
Encumbrances	0
Original Land	594,000
Original Building	6,748,000
Costs Capitalized Subsequent to Acquisition	7,819,000
Land and improvements	802,000
Buildings and Improvements	14,359,000
Land Held for Development	0
Development and Construction in Progress	0
Total	15,161,000
Accumulated Depreciation	10,665,000
Year(s) built / renovated	1985
Depreciable Lives (Years)	-1	[2]
91 Hartwell Avenue [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	91 Hartwell Avenue
Type	Office
Location	Lexington, MA
Encumbrances	0
Original Land	784,000
Original Building	6,464,000
Costs Capitalized Subsequent to Acquisition	6,747,000
Land and improvements	941,000
Buildings and Improvements	13,054,000
Land Held for Development	0
Development and Construction in Progress	0
Total	13,995,000
Accumulated Depreciation	7,655,000
Year(s) built / renovated	1985
Depreciable Lives (Years)	-1	[2]
181 Spring Street [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	181 Spring Street
Type	Office
Location	Lexington, MA
Encumbrances	0
Original Land	1,066,000
Original Building	9,520,000
Costs Capitalized Subsequent to Acquisition	1,950,000
Land and improvements	1,160,000
Buildings and Improvements	11,376,000
Land Held for Development	0
Development and Construction in Progress	0
Total	12,536,000
Accumulated Depreciation	4,306,000
Year(s) built / renovated	1999
Depreciable Lives (Years)	-1	[2]
195 West Street [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	195 West Street
Type	Office
Location	Waltham, MA
Encumbrances	0
Original Land	1,611,000
Original Building	6,652,000
Costs Capitalized Subsequent to Acquisition	4,267,000
Land and improvements	1,858,000
Buildings and Improvements	10,672,000
Land Held for Development	0
Development and Construction in Progress	0
Total	12,530,000
Accumulated Depreciation	6,626,000
Year(s) built / renovated	1990
Depreciable Lives (Years)	-1	[2]
33 Hayden Avenue [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	33 Hayden Avenue
Type	Office
Location	Lexington, MA
Encumbrances	0
Original Land	266,000
Original Building	3,234,000
Costs Capitalized Subsequent to Acquisition	8,787,000
Land and improvements	425,000
Buildings and Improvements	11,862,000
Land Held for Development	0
Development and Construction in Progress	0
Total	12,287,000
Accumulated Depreciation	5,558,000
Year(s) built / renovated	1979
Depreciable Lives (Years)	-1	[2]
145 Broadway (formerly Eleven Cambridge Center) [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	145 Broadway (formerly Eleven Cambridge Center)
Type	Office
Location	Cambridge, MA
Encumbrances	0
Original Land	121,000
Original Building	5,535,000
Costs Capitalized Subsequent to Acquisition	5,278,000
Land and improvements	324,000
Buildings and Improvements	10,610,000
Land Held for Development	0
Development and Construction in Progress	0
Total	10,934,000
Accumulated Depreciation	7,566,000
Year(s) built / renovated	1984
Depreciable Lives (Years)	-1	[2]
7501 Boston Boulevard, Building Seven [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	7501 Boston Boulevard, Building Seven
Type	Office
Location	Springfield, VA
Encumbrances	0
Original Land	665,000
Original Building	9,273,000
Costs Capitalized Subsequent to Acquisition	520,000
Land and improvements	791,000
Buildings and Improvements	9,667,000
Land Held for Development	0
Development and Construction in Progress	0
Total	10,458,000
Accumulated Depreciation	4,113,000
Year(s) built / renovated	1997
Depreciable Lives (Years)	-1	[2]
7435 Boston Boulevard, Building One [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	7435 Boston Boulevard, Building One
Type	Office
Location	Springfield, VA
Encumbrances	0
Original Land	392,000
Original Building	3,822,000
Costs Capitalized Subsequent to Acquisition	3,798,000
Land and improvements	659,000
Buildings and Improvements	7,353,000
Land Held for Development	0
Development and Construction in Progress	0
Total	8,012,000
Accumulated Depreciation	5,794,000
Year(s) built / renovated	1982
Depreciable Lives (Years)	-1	[2]
7450 Boston Boulevard, Building Three [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	7450 Boston Boulevard, Building Three
Type	Office
Location	Springfield, VA
Encumbrances	0
Original Land	1,165,000
Original Building	4,681,000
Costs Capitalized Subsequent to Acquisition	1,880,000
Land and improvements	1,430,000
Buildings and Improvements	6,296,000
Land Held for Development	0
Development and Construction in Progress	0
Total	7,726,000
Accumulated Depreciation	3,056,000
Year(s) built / renovated	1987
Depreciable Lives (Years)	-1	[2]
8000 Grainger Court, Building Five [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	8000 Grainger Court, Building Five
Type	Office
Location	Springfield, VA
Encumbrances	0
Original Land	366,000
Original Building	4,282,000
Costs Capitalized Subsequent to Acquisition	2,684,000
Land and improvements	601,000
Buildings and Improvements	6,731,000
Land Held for Development	0
Development and Construction in Progress	0
Total	7,332,000
Accumulated Depreciation	5,142,000
Year(s) built / renovated	1984
Depreciable Lives (Years)	-1	[2]
453 Ravendale Drive [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	453 Ravendale Drive
Type	Office
Location	Mountain View, CA
Encumbrances	0
Original Land	5,477,000
Original Building	1,090,000
Costs Capitalized Subsequent to Acquisition	230,000
Land and improvements	5,477,000
Buildings and Improvements	1,320,000
Land Held for Development	0
Development and Construction in Progress	0
Total	6,797,000
Accumulated Depreciation	213,000
Year(s) built / renovated	1977
Depreciable Lives (Years)	-1	[2]
7500 Boston Boulevard, Building Six [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	7500 Boston Boulevard, Building Six
Type	Office
Location	Springfield, VA
Encumbrances	0
Original Land	138,000
Original Building	3,749,000
Costs Capitalized Subsequent to Acquisition	2,538,000
Land and improvements	406,000
Buildings and Improvements	6,019,000
Land Held for Development	0
Development and Construction in Progress	0
Total	6,425,000
Accumulated Depreciation	4,265,000
Year(s) built / renovated	1985
Depreciable Lives (Years)	-1	[2]
7601 Boston Boulevard, Building Eight [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	7601 Boston Boulevard, Building Eight
Type	Office
Location	Springfield, VA
Encumbrances	0
Original Land	200,000
Original Building	878,000
Costs Capitalized Subsequent to Acquisition	4,826,000
Land and improvements	551,000
Buildings and Improvements	5,353,000
Land Held for Development	0
Development and Construction in Progress	0
Total	5,904,000
Accumulated Depreciation	3,688,000
Year(s) built / renovated	1986
Depreciable Lives (Years)	-1	[2]
250 Binney Street (formerly Fourteen Cambridge Center) [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	250 Binney Street (formerly Fourteen Cambridge Center)
Type	Office
Location	Cambridge, MA
Encumbrances	0
Original Land	110,000
Original Building	4,483,000
Costs Capitalized Subsequent to Acquisition	1,223,000
Land and improvements	273,000
Buildings and Improvements	5,543,000
Land Held for Development	0
Development and Construction in Progress	0
Total	5,816,000
Accumulated Depreciation	3,943,000
Year(s) built / renovated	1983
Depreciable Lives (Years)	-1	[2]
7300 Boston Boulevard, Building Thirteen [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	7300 Boston Boulevard, Building Thirteen
Type	Office
Location	Springfield, VA
Encumbrances	0
Original Land	608,000
Original Building	4,773,000
Costs Capitalized Subsequent to Acquisition	230,000
Land and improvements	661,000
Buildings and Improvements	4,950,000
Land Held for Development	0
Development and Construction in Progress	0
Total	5,611,000
Accumulated Depreciation	4,229,000
Year(s) built / renovated	2002
Depreciable Lives (Years)	-1	[2]
8000 Corporate Court, Building Eleven [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	8000 Corporate Court, Building Eleven
Type	Office
Location	Springfield, VA
Encumbrances	0
Original Land	136,000
Original Building	3,071,000
Costs Capitalized Subsequent to Acquisition	1,611,000
Land and improvements	775,000
Buildings and Improvements	4,043,000
Land Held for Development	0
Development and Construction in Progress	0
Total	4,818,000
Accumulated Depreciation	2,494,000
Year(s) built / renovated	1989
Depreciable Lives (Years)	-1	[2]
7375 Boston Boulevard, Building Ten [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	7375 Boston Boulevard, Building Ten
Type	Office
Location	Springfield, VA
Encumbrances	0
Original Land	23,000
Original Building	2,685,000
Costs Capitalized Subsequent to Acquisition	1,003,000
Land and improvements	93,000
Buildings and Improvements	3,618,000
Land Held for Development	0
Development and Construction in Progress	0
Total	3,711,000
Accumulated Depreciation	2,362,000
Year(s) built / renovated	1988
Depreciable Lives (Years)	-1	[2]
7374 Boston Boulevard, Building Four [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	7374 Boston Boulevard, Building Four
Type	Office
Location	Springfield, VA
Encumbrances	0
Original Land	241,000
Original Building	1,605,000
Costs Capitalized Subsequent to Acquisition	1,729,000
Land and improvements	398,000
Buildings and Improvements	3,177,000
Land Held for Development	0
Development and Construction in Progress	0
Total	3,575,000
Accumulated Depreciation	2,235,000
Year(s) built / renovated	1984
Depreciable Lives (Years)	-1	[2]
7451 Boston Boulevard, Building Two [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	7451 Boston Boulevard, Building Two
Type	Office
Location	Springfield, VA
Encumbrances	0
Original Land	249,000
Original Building	1,542,000
Costs Capitalized Subsequent to Acquisition	1,661,000
Land and improvements	613,000
Buildings and Improvements	2,839,000
Land Held for Development	0
Development and Construction in Progress	0
Total	3,452,000
Accumulated Depreciation	2,135,000
Year(s) built / renovated	1982
Depreciable Lives (Years)	-1	[2]
32 Hartwell Avenue [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	32 Hartwell Avenue
Type	Office
Location	Lexington, MA
Encumbrances	0
Original Land	168,000
Original Building	1,943,000
Costs Capitalized Subsequent to Acquisition	809,000
Land and improvements	314,000
Buildings and Improvements	2,606,000
Land Held for Development	0
Development and Construction in Progress	0
Total	2,920,000
Accumulated Depreciation	1,685,000
Year(s) built / renovated	1968-1979/1987
Depreciable Lives (Years)	-1	[2]
164 Lexington Road [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	164 Lexington Road
Type	Office
Location	Billerica, MA
Encumbrances	0
Original Land	592,000
Original Building	1,370,000
Costs Capitalized Subsequent to Acquisition	319,000
Land and improvements	643,000
Buildings and Improvements	1,638,000
Land Held for Development	0
Development and Construction in Progress	0
Total	2,281,000
Accumulated Depreciation	745,000
Year(s) built / renovated	1982
Depreciable Lives (Years)	-1	[2]
17 Hartwell Avenue [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	17 Hartwell Avenue
Type	Office
Location	Lexington, MA
Encumbrances	0
Original Land	26,000
Original Building	150,000
Costs Capitalized Subsequent to Acquisition	1,018,000
Land and improvements	65,000
Buildings and Improvements	1,113,000
Land Held for Development	16,000
Development and Construction in Progress	0
Total	1,194,000
Accumulated Depreciation	714,000
Year(s) built / renovated	1968
Depreciable Lives (Years)	-1	[2]
Residences on The Avenue, Two Two Two One I St., NW [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	Residences on The Avenue, 2221 I St., NW
Type	Residential
Location	Washington, DC
Encumbrances	0
Original Land	0
Original Building	119,874,000
Costs Capitalized Subsequent to Acquisition	-28,000
Land and improvements	0
Buildings and Improvements	119,846,000
Land Held for Development	0
Development and Construction in Progress	0
Total	119,846,000
Accumulated Depreciation	11,039,000
Year(s) built / renovated	2011
Depreciable Lives (Years)	-1	[2]
The Avant at Reston Town Center Residential [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	The Avant at Reston Town Center
Type	Residential
Location	Reston, VA
Encumbrances	0
Original Land	20,350,000
Original Building	91,995,000
Costs Capitalized Subsequent to Acquisition	0
Land and improvements	20,350,000
Buildings and Improvements	91,995,000
Land Held for Development	0
Development and Construction in Progress	0
Total	112,345,000
Accumulated Depreciation	2,397,000
Year(s) built / renovated	2014
Depreciable Lives (Years)	-1	[2]
The Lofts At Atlantic Wharf [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	The Lofts at Atlantic Wharf
Type	Residential
Location	Boston, MA
Encumbrances	0
Original Land	3,529,000
Original Building	54,891,000
Costs Capitalized Subsequent to Acquisition	1,543,000
Land and improvements	3,529,000
Buildings and Improvements	56,434,000
Land Held for Development	0
Development and Construction in Progress	0
Total	59,963,000
Accumulated Depreciation	5,093,000
Year(s) built / renovated	2011
Depreciable Lives (Years)	-1	[2]
Boston Marriott Cambridge (formerly Cambridge Center Marriott) [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	Boston Marriott Cambridge (formerly Cambridge Center Marriott)
Type	Hotel
Location	Cambridge, MA
Encumbrances	0
Original Land	478,000
Original Building	37,918,000
Costs Capitalized Subsequent to Acquisition	36,160,000
Land and improvements	1,201,000
Buildings and Improvements	73,355,000
Land Held for Development	0
Development and Construction in Progress	0
Total	74,556,000
Accumulated Depreciation	47,196,000
Year(s) built / renovated	1986
Depreciable Lives (Years)	-1	[2]
Cambridge Center East Garage [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	Cambridge Center East Garage
Type	Garage
Location	Cambridge, MA
Encumbrances	0
Original Land	0
Original Building	35,035,000
Costs Capitalized Subsequent to Acquisition	1,561,000
Land and improvements	103,000
Buildings and Improvements	36,493,000
Land Held for Development	0
Development and Construction in Progress	0
Total	36,596,000
Accumulated Depreciation	7,647,000
Year(s) built / renovated	1984
Depreciable Lives (Years)	-1	[2]
Cambridge Center West Garage [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	Cambridge Center West Garage
Type	Garage
Location	Cambridge, MA
Encumbrances	0
Original Land	1,256,000
Original Building	15,697,000
Costs Capitalized Subsequent to Acquisition	1,571,000
Land and improvements	1,434,000
Buildings and Improvements	17,090,000
Land Held for Development	0
Development and Construction in Progress	0
Total	18,524,000
Accumulated Depreciation	3,815,000
Year(s) built / renovated	2006
Depreciable Lives (Years)	-1	[2]
Cambridge Center North Garage [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	Cambridge Center North Garage
Type	Garage
Location	Cambridge, MA
Encumbrances	0
Original Land	1,163,000
Original Building	11,633,000
Costs Capitalized Subsequent to Acquisition	2,770,000
Land and improvements	1,579,000
Buildings and Improvements	13,987,000
Land Held for Development	0
Development and Construction in Progress	0
Total	15,566,000
Accumulated Depreciation	8,531,000
Year(s) built / renovated	1990
Depreciable Lives (Years)	-1	[2]
Salesforce Tower [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	Salesforce Tower
Type	Development
Location	San Francisco, CA
Encumbrances	0
Original Land	0
Original Building	0
Costs Capitalized Subsequent to Acquisition	345,303,000
Land and improvements	0
Buildings and Improvements	0
Land Held for Development	0
Development and Construction in Progress	345,303,000
Total	345,303,000
Accumulated Depreciation	0
Year(s) built / renovated	N/A
Depreciable Lives (Years)	N/A
601 Massachusetts Avenue [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	601 Massachusetts Avenue
Type	Development
Location	Washington, DC
Costs Capitalized Subsequent to Acquisition	189,173,000
Land and improvements	12,000
Buildings and Improvements	0
Land Held for Development	0
Development and Construction in Progress	189,161,000
Total	189,173,000
Accumulated Depreciation	0
Year(s) built / renovated	N/A
Depreciable Lives (Years)	N/A
535 Mission Street [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	535 Mission Street
Type	Development
Location	San Francisco, CA
Costs Capitalized Subsequent to Acquisition	171,096,000
Land and improvements	10,789,000
Buildings and Improvements	39,307,000
Development and Construction in Progress	121,000,000
Total	171,096,000
Accumulated Depreciation	225,000
Year(s) built / renovated	N/A
Depreciable Lives (Years)	N/A
10 CityPoint [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	10 CityPoint
Type	Development
Location	Waltham, MA
Costs Capitalized Subsequent to Acquisition	21,533,000
Land and improvements	163,000
Buildings and Improvements	0
Land Held for Development	0
Development and Construction in Progress	21,370,000
Total	21,533,000
Accumulated Depreciation	0
Year(s) built / renovated	N/A
Depreciable Lives (Years)	N/A
690 Folsom Street [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	690 Folsom Street
Type	Development
Location	San Francisco, CA
Costs Capitalized Subsequent to Acquisition	13,237,000
Land and improvements	1,777,000
Buildings and Improvements	6,546,000
Land Held for Development	0
Development and Construction in Progress	4,914,000
Total	13,237,000
Accumulated Depreciation	11,000
Year(s) built / renovated	N/A
Depreciable Lives (Years)	N/A
Springfield Metro Center [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	Springfield Metro Center
Type	Land
Location	Springfield, VA
Encumbrances	0
Original Land	0
Original Building	0
Costs Capitalized Subsequent to Acquisition	32,445,000
Land and improvements	0
Buildings and Improvements	0
Land Held for Development	32,445,000
Development and Construction in Progress	0
Total	32,445,000
Accumulated Depreciation	0
Year(s) built / renovated	N/A
Depreciable Lives (Years)	N/A
Reston Signature Site [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	Reston Signature Site
Type	Land
Location	Reston, VA
Encumbrances	0
Original Land	0
Original Building	0
Costs Capitalized Subsequent to Acquisition	30,256,000
Land and improvements	0
Buildings and Improvements	0
Land Held for Development	30,256,000
Development and Construction in Progress	0
Total	30,256,000
Accumulated Depreciation	0
Year(s) built / renovated	N/A
Depreciable Lives (Years)	N/A
Plaza at Almaden [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	Plaza at Almaden
Type	Land
Location	San Jose, CA
Encumbrances	0
Original Land	0
Original Building	0
Costs Capitalized Subsequent to Acquisition	29,006,000
Land and improvements	0
Buildings and Improvements	0
Land Held for Development	29,006,000
Development and Construction in Progress	0
Total	29,006,000
Accumulated Depreciation	0
Year(s) built / renovated	N/A
Depreciable Lives (Years)	N/A
Tower Oaks Master Plan [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	Tower Oaks Master Plan
Type	Land
Location	Rockville, MD
Encumbrances	0
Original Land	0
Original Building	0
Costs Capitalized Subsequent to Acquisition	28,919,000
Land and improvements	0
Buildings and Improvements	0
Land Held for Development	28,919,000
Development and Construction in Progress	0
Total	28,919,000
Accumulated Depreciation	0
Year(s) built / renovated	N/A
Depreciable Lives (Years)	N/A
214 Third Avenue (formerly Prospect Hill) [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	214 Third Avenue (formerly Prospect Hill)
Type	Land
Location	Waltham, MA
Encumbrances	0
Original Land	0
Original Building	0
Costs Capitalized Subsequent to Acquisition	23,255,000
Land and improvements	0
Buildings and Improvements	132,000
Land Held for Development	12,956,000
Development and Construction in Progress	10,167,000
Total	23,255,000
Accumulated Depreciation	0
Year(s) built / renovated	N/A
Depreciable Lives (Years)	N/A
Washingtonian North [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	Washingtonian North
Type	Land
Location	Gaithersburg, MD
Encumbrances	0
Original Land	0
Original Building	0
Costs Capitalized Subsequent to Acquisition	18,813,000
Land and improvements	0
Buildings and Improvements	0
Land Held for Development	18,813,000
Development and Construction in Progress	0
Total	18,813,000
Accumulated Depreciation	0
Year(s) built / renovated	N/A
Depreciable Lives (Years)	N/A
6601 and 6605 Springfield Center Drive [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	6601 & 6605 Springfield Center Drive
Type	Land
Location	Springfield, VA
Encumbrances	0
Original Land	0
Original Building	0
Costs Capitalized Subsequent to Acquisition	13,866,000
Land and improvements	0
Buildings and Improvements	0
Land Held for Development	13,866,000
Development and Construction in Progress	0
Total	13,866,000
Accumulated Depreciation	0
Year(s) built / renovated	N/A
Depreciable Lives (Years)	N/A
103 4th Avenue [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	103 Fourth Avenue
Type	Land
Location	Waltham, MA
Encumbrances	0
Original Land	0
Original Building	0
Costs Capitalized Subsequent to Acquisition	11,920,000
Land and improvements	0
Buildings and Improvements	0
Land Held for Development	11,920,000
Development and Construction in Progress	0
Total	11,920,000
Accumulated Depreciation	0
Year(s) built / renovated	N/A
Depreciable Lives (Years)	N/A
Reston Gateway [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	Reston Gateway
Type	Land
Location	Reston, VA
Encumbrances	0
Original Land	0
Original Building	0
Costs Capitalized Subsequent to Acquisition	9,933,000
Land and improvements	0
Buildings and Improvements	0
Land Held for Development	9,933,000
Development and Construction in Progress	0
Total	9,933,000
Accumulated Depreciation	0
Year(s) built / renovated	N/A
Depreciable Lives (Years)	N/A
Reston Eastgate [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	Reston Eastgate
Type	Land
Location	Reston, VA
Encumbrances	0
Original Land	0
Original Building	0
Costs Capitalized Subsequent to Acquisition	8,817,000
Land and improvements	0
Buildings and Improvements	0
Land Held for Development	8,817,000
Development and Construction in Progress	0
Total	8,817,000
Accumulated Depreciation	0
Year(s) built / renovated	N/A
Depreciable Lives (Years)	N/A
Crane Meadow [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	Crane Meadow
Type	Land
Location	Marlborough, MA
Encumbrances	0
Original Land	0
Original Building	0
Costs Capitalized Subsequent to Acquisition	8,726,000
Land and improvements	0
Buildings and Improvements	0
Land Held for Development	8,726,000
Development and Construction in Progress	0
Total	8,726,000
Accumulated Depreciation	0
Year(s) built / renovated	N/A
Depreciable Lives (Years)	N/A
Broad Run Business Park [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	Broad Run Business Park
Type	Land
Location	Loudoun County, VA
Encumbrances	0
Original Land	0
Original Building	0
Costs Capitalized Subsequent to Acquisition	6,311,000
Land and improvements	0
Buildings and Improvements	0
Land Held for Development	6,311,000
Development and Construction in Progress	0
Total	6,311,000
Accumulated Depreciation	0
Year(s) built / renovated	N/A
Depreciable Lives (Years)	N/A
20 CityPoint [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	20 CityPoint
Type	Land
Location	Waltham, MA
Encumbrances	0
Original Land	0
Original Building	0
Costs Capitalized Subsequent to Acquisition	4,801,000
Land and improvements	0
Buildings and Improvements	0
Land Held for Development	4,801,000
Development and Construction in Progress	0
Total	4,801,000
Accumulated Depreciation	0
Year(s) built / renovated	N/A
Depreciable Lives (Years)	N/A
Cambridge Master Plan [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	Cambridge Master Plan
Type	Land
Location	Cambridge, MA
Encumbrances	0
Original Land	0
Original Building	0
Costs Capitalized Subsequent to Acquisition	3,527,000
Land and improvements	0
Buildings and Improvements	0
Land Held for Development	3,527,000
Development and Construction in Progress	0
Total	3,527,000
Accumulated Depreciation	0
Year(s) built / renovated	N/A
Depreciable Lives (Years)	N/A
North First Master Plan [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	North First Master Plan
Type	Land
Location	San Jose, CA
Encumbrances	0
Original Land	0
Original Building	0
Costs Capitalized Subsequent to Acquisition	1,664,000
Land and improvements	0
Buildings and Improvements	0
Land Held for Development	1,664,000
Development and Construction in Progress	0
Total	1,664,000
Accumulated Depreciation	0
Year(s) built / renovated	N/A
Depreciable Lives (Years)	N/A
Four Two Five Fourth Street [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	425 Fourth Street
Type	Land
Location	San Francisco, CA
Encumbrances	0
Original Land	0
Original Building	0
Costs Capitalized Subsequent to Acquisition	1,261,000
Land and improvements	0
Buildings and Improvements	0
Land Held for Development	1,261,000
Development and Construction in Progress	0
Total	1,261,000
Accumulated Depreciation	0
Year(s) built / renovated	N/A
Depreciable Lives (Years)	N/A
30 Shattuck Road [Member]
Real Estate and Accumulated Depreciation [Line Items]
Real Estate And Accumulated Depreciation Name of Property	30 Shattuck Road
Type	Land
Location	Andover, MA
Encumbrances	0
Original Land	0
Original Building	0
Costs Capitalized Subsequent to Acquisition	1,213,000
Land and improvements	0
Buildings and Improvements	0
Land Held for Development	1,213,000
Development and Construction in Progress	0
Total	1,213,000
Accumulated Depreciation	$0
Year(s) built / renovated	N/A
Depreciable Lives (Years)	N/A
Minimum [Member]
Real Estate and Accumulated Depreciation [Line Items]
Life Used for Depreciation	life of the lease
[1]	(2)Includes the unamortized balance of the historical fair value adjustment totaling approximately $138.7 million.
[2]	(1)Depreciation of the buildings and improvements are calculated over lives ranging from the life of the lease to 40 years.

Real_Estate_and_Accumulatied_D1

Real Estate and Accumulatied Deprecitation Reconcilation Real estate and accumulated depreciation (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
SEC Schedule III, Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]
Real Estate, Gross, Starting Balance	$18,953,601	$14,869,887	$13,363,113
Additions to/improvements of real estate	594,296	4,410,622	1,602,583
Assets sold/written-off	-339,480	-326,908	-95,809
Real Estate, Gross, Ending Balance	19,208,417	18,953,601	14,869,887
SEC Schedule III, Reconciliation of Real Estate Accumulated Depreciation [Roll Forward]
Real Estate Accumulated Depreciation, Beginning Balance	3,145,701	2,919,479	2,626,324
Depreciation expense	456,176	419,908	367,625
Assets sold/written-off	-71,899	-193,686	-74,470
Real Estate Accumulated Depreciation, Ending Balance	$3,529,978	$3,145,701	$2,919,479

Uncategorized_Items

Uncategorized Items
[us-gaap_NumberOfRealEstateProperties]	7	135,000