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The equity interest of noncontrolling shareholders, partners or other equity holders in consolidated entity.

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Mortgage loan related to property sales in noncash investing and financing activities.

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Net rentable area for properties owned.

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The cash outflow from the acquisition of a piece of land and anything permanently fixed to it, including buildings, structures on it and so forth, and payments to develop real estate assets by adding improvements on or to a parcel of land. Such improvements may include drainage, utilities, subdividing, access, buildings, and any combination of these elements. Including real estate intended to generate income for the owner; excluding real estate acquired for use by the owner.

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Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

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Real Estate (Narrative) (Details) $ in Thousands										3 Months Ended									7 Months Ended	9 Months Ended		12 Months Ended
Real Estate (Narrative) (Details) $ in Thousands	Dec. 17, 2015 USD ($) ft² a spaces Buildings	Oct. 01, 2015 USD ($) a	Sep. 18, 2015 USD ($) ft²	Aug. 13, 2015 USD ($) ft²	Jul. 31, 2015 USD ($) yr	Mar. 17, 2015 USD ($) ft² apartments	Feb. 19, 2015 USD ($) a	May. 31, 2013 USD ($) ft² partners	Apr. 10, 2013 USD ($) ft² Buildings	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Mar. 16, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)	Jun. 30, 2014 USD ($)	Mar. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Sep. 17, 2015 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2015 USD ($) yr	Dec. 16, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 02, 2015 ft²	Nov. 01, 2015 ft²	Sep. 10, 2015 ft²	Aug. 14, 2015 ft²	Jul. 23, 2015 ft² apartments	May. 01, 2015 ft²
Real Estate Properties [Line Items]
Gain on consolidation of joint ventures																						$ 0		$ 0	$ 385,991
Payments to Acquire and Develop Real Estate																						0		4,670	522,900
Total revenue										$ 624,240	$ 629,884	$ 618,221	$ 618,476		$ 613,707	$ 618,803	$ 589,794	$ 574,694				2,490,821		2,396,998	2,135,539
Earnings																						849,365		801,822	678,120
Proceeds from sales of real estate																						602,600		419,864	250,078
Impairment loss																						0		0	8,306
Gains on sales of real estate																						$ 375,895		168,039	0
Mountain View Research And Technology Parks [Member]
Real Estate Properties [Line Items]
Gain on consolidation of joint ventures									$ 26,500
Total revenue																					$ 16,700
Earnings																					$ 400
Aggregate purchase price									$ 233,084
Mountain View Research Park
Real Estate Properties [Line Items]
Number of real estate properties \| Buildings									16
Net Rentable Area \| ft²									604,000
Mountain View Technology [Member]
Real Estate Properties [Line Items]
Number of real estate properties \| Buildings									7
Net Rentable Area \| ft²									135,000
767 Fifth Avenue (the General Motors Building)
Real Estate Properties [Line Items]
Gain on consolidation of joint ventures								$ 359,500
Total revenue																			$ 168,400
Earnings																			$ 8,400
Net Rentable Area \| ft²								1,800,000
Number of Joint Venture Partners \| partners								2
Ownership Percentage								40.00%
Washingtonian North land parcel [Member]
Real Estate Properties [Line Items]
Proceeds from sales of real estate		$ 13,800					$ 8,400
Sale Price Of Sold Property		$ 13,300					$ 8,700
Area of Land \| a		18.3					27
Gains on sales of real estate		$ 2,000					$ 3,500
Land sold (in acres) \| a		5.8					8.5
Innovation Place [Member]
Real Estate Properties [Line Items]
Earnings																							$ (3,500)	(3,100)	400
Proceeds from sales of real estate	$ 199,300
Net Rentable Area \| ft²	574,000
Parking spaces \| spaces	1,699
Sale Price Of Sold Property	$ 207,000
Area of Land \| a	26
Gains on sales of real estate	$ 79,100
Residences on The Avenue [Member]
Real Estate Properties [Line Items]
Earnings														$ 1,100										2,700	4,400
Proceeds from sales of real estate						$ 192,500
Sale Price Of Sold Property						$ 196,000
Number of apartment units \| apartments						335
Gains on sales of real estate						$ 91,400
Rental revenue supoprt payments to buyer maximum						$ 6,000
505 9th Street [Member] \| Consolidated Properties [Member]
Real Estate Properties [Line Items]
Earnings																				$ 2,300				2,300	1,900
Ownership Percentage by Parent			50.00%
Proceeds from sales of real estate			$ 194,600
Net Rentable Area \| ft²			322,000
Sale Price Of Sold Property			$ 318,000
Mortgage Loan Related to Property Sales			117,000
Gains on sales of real estate			199,500
Reservoir Place North [Member]
Real Estate Properties [Line Items]
Net Rentable Area \| ft²																															73,000
Cambridge Residential / 88 Ames
Real Estate Properties [Line Items]
Net Rentable Area \| ft²																														164,000
Area of Land \| ft²				8,700
Number of apartment units \| apartments																														274
Payments to Acquire Land				$ 2,000
Reston Signature Site
Real Estate Properties [Line Items]
Net Rentable Area \| ft²																														514,000
Number of apartment units \| apartments																														508
601 Massachusetts Avenue
Real Estate Properties [Line Items]
Net Rentable Area \| ft²																													478,000
690 Folsom Street
Real Estate Properties [Line Items]
Net Rentable Area \| ft²																										26,000
The Point (formerly 99 Third Avenue Retail)
Real Estate Properties [Line Items]
Net Rentable Area \| ft²																												16,000
535 Mission Street
Real Estate Properties [Line Items]
Net Rentable Area \| ft²																											307,000
Retail Site [Member] \| Residences on The Avenue [Member]
Real Estate Properties [Line Items]
Net Rentable Area \| ft²						50,000
Retail Site [Member] \| Cambridge Residential / 88 Ames
Real Estate Properties [Line Items]
Net Rentable Area \| ft²																														9,000
Retail Site [Member] \| Reston Signature Site
Real Estate Properties [Line Items]
Net Rentable Area \| ft²																														24,000
Occupied [Member] \| Innovation Place [Member]
Real Estate Properties [Line Items]
Number of real estate properties \| Buildings	1
Vacant [Member] \| Innovation Place [Member]
Real Estate Properties [Line Items]
Number of real estate properties \| Buildings	3
Net Rentable Area \| ft²	463,000
Future potential [Member] \| Innovation Place [Member]
Real Estate Properties [Line Items]
Net Rentable Area \| ft²	537,000
Parking structures \| Buildings	2
Parking spaces \| spaces	3,000
Clarendon Garage [Member]
Real Estate Properties [Line Items]
Ground Lease Term (in years) \| yr					99																	99
Operating Lease Ground Rent Expense					$ 5,000
Maximum [Member] \| Clarendon Garage [Member]
Real Estate Properties [Line Items]
Operating Lease Ground Rent Expense					$ 37,000
Previous ground lease remaining term [Member] \| Clarendon Garage [Member]
Real Estate Properties [Line Items]
Ground Lease Term (in years) \| yr					45
Consolidated Entity Excluding Variable Interest Entities (VIE) [Member] \| 505 9th Street [Member] \| Consolidated Properties [Member]
Real Estate Properties [Line Items]
Proceeds from sales of real estate			97,300
Gains on sales of real estate			101,100
Boston Properties Limited Partnership [Member]
Real Estate Properties [Line Items]
Gain on consolidation of joint ventures																						$ 0		0	385,991
Payments to Acquire and Develop Real Estate																						0		4,670	522,900
Total revenue										$ 624,240	$ 629,884	$ 618,221	$ 618,476		$ 613,707	$ 618,803	$ 589,794	$ 574,694				2,490,821		2,396,998	2,135,539
Earnings																						857,358		810,331	690,073
Proceeds from sales of real estate																						602,600		419,864	250,078
Impairment loss																						0		0	4,401
Gains on sales of real estate																						$ 377,093		$ 174,686	$ 0
Boston Properties Limited Partnership [Member] \| Innovation Place [Member]
Real Estate Properties [Line Items]
Gains on sales of real estate	$ 80,100
Boston Properties Limited Partnership [Member] \| 505 9th Street [Member] \| Consolidated Properties [Member]
Real Estate Properties [Line Items]
Gains on sales of real estate			$ 199,700