Investments in Unconsolidated Joint Ventures (Statements of Operations of the Joint Ventures) (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Schedule of Equity Method Investments [Line Items]
Total revenue		$ 705,157	$ 686,284	$ 664,484	$ 661,151	$ 655,229	$ 657,712	$ 656,907	$ 632,228	$ 2,717,076	$ 2,602,076	$ 2,550,820
Expenses
Depreciation and amortization										645,649	617,547	694,403
Total expenses										1,791,579	1,693,966	1,723,253
Other income (expense)
Interest expense										(378,168)	(374,481)	(412,849)
Gains on sales of real estate										182,356	7,663	80,606
Net income										712,563	562,481	569,977
Income from unconsolidated joint ventures										2,222	11,232	8,074
Gain on sale of investment in unconsolidated joint venture										0	0	59,370
Straight-line rent adjustments										51,900	54,800	31,700
Mortgage and notes payable, net		2,964,572				2,979,281				2,964,572	2,979,281
Above and below market rent adjustments, net										23,800	23,500	30,200
Gain on sale of investment in unconsolidated joint venture										0	0	55,707
Unconsolidated Joint Ventures [Member]
Schedule of Equity Method Investments [Line Items]
Total revenue	[1]									271,951	222,517	177,182
Expenses
Operating										106,610	90,542	76,741
Depreciation and amortization	[2]									103,079	57,079	44,989
Total expenses										209,689	147,621	121,730
Other income (expense)
Interest expense										(71,308)	(46,371)	(34,016)
Gains on sales of real estate										16,951	0	0
Net income										7,905	28,525	21,436
Company's share of net income										8,084	18,439	9,873
Basis differential	[3]									(5,862)	(7,207)	(1,799)
Income from unconsolidated joint ventures										2,222	11,232	8,074
Gain on sale of investment in unconsolidated joint venture										0	0	59,370
Straight-line rent adjustments										15,900	21,700	18,100
Mortgage and notes payable, net		$ 2,017,609				$ 1,437,440				2,017,609	1,437,440
The Metropolitan Square Associates LLC [Member]
Expenses
Depreciation and amortization										22,400
The Metropolitan Square Associates LLC [Member] | Boston Properties, Inc. [Member]
Expenses
Depreciation and amortization										4,500
Colorado Center [Member]
Other income (expense)
Straight-line rent adjustments										2,400	1,900	1,400
Above and below market rent adjustments, net										$ 1,600	$ 2,900	$ 900

[1] Includes straight-line rent adjustments of approximately $15.9 million, $21.7 million and $18.1 million for the years ended December 31, 2018, 2017 and 2016, respectively.

[2] During the year ended December 31, 2018, the joint venture that owns Metropolitan Square in Washington, DC, commenced a renovation project and recorded accelerated depreciation expense of approximately $22.4 million related to the remaining book value of the assets to be replaced. The Company's share of the accelerated depreciation expense totaled approximately $4.5 million.

[3] Includes straight-line rent adjustments of approximately $2.4 million, $1.9 million and $1.4 million for the years ended December 31, 2018, 2017 and 2016, respectively. Also includes net above-/below-market rent adjustments of approximately $1.6 million, $2.9 million and $0.9 million for the years ended December 31, 2018, 2017 and 2016, respectively.