Investments in Unconsolidated Joint Ventures (Statements of Operations of the Joint Ventures) (Details) - USD ($) $ in Thousands		3 Months Ended			9 Months Ended
		Sep. 30, 2018		Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Schedule of Equity Method Investments [Line Items]
Total revenue		$ 686,284		$ 657,712	$ 2,011,919	$ 1,946,847
Expenses
Depreciation and amortization		157,996		152,164	480,210	463,288
Total expenses		447,920		423,983	1,334,779	1,268,203
Operating income		238,364		233,729	677,140	678,644
Other expense
Interest expense		95,366		92,032	277,790	282,709
Net income (loss)		150,445		147,704	527,322	430,278
Income (loss) from unconsolidated joint ventures		(4,313)		843	(3,083)	7,035
Unconsolidated Joint Ventures [Member]
Schedule of Equity Method Investments [Line Items]
Total revenue	[1]	76,177		55,516	189,759	166,139
Expenses
Operating		29,522		23,128	75,239	67,310
Depreciation and amortization		44,993	[2]	16,440	74,245	44,973
Total expenses		74,515		39,568	149,484	112,283
Operating income		1,662		15,948	40,275	53,856
Other expense
Interest expense		19,861		13,088	48,993	31,815
Net income (loss)		(18,199)		2,860	(8,718)	22,041
Company's share of net income (loss)		(2,858)		2,909	1,073	11,576
Basis differential	[3]	(1,455)		(2,066)	(4,156)	(4,541)
Income (loss) from unconsolidated joint ventures		(4,313)		843	(3,083)	7,035
Straight-line rent adjustments		4,800		5,100	9,800	16,400
The Metropolitan Square Associates LLC [Member]
Expenses
Depreciation and amortization		22,400
The Metropolitan Square Associates LLC [Member] | BXP [Member]
Expenses
Depreciation and amortization		4,500
Colorado Center [Member] | Unconsolidated Joint Ventures [Member]
Other expense
Straight-line rent adjustments		500		700	1,900	2,200
"Above" and "below" market rent adjustments, net		$ 400		$ 400	$ 1,200	$ 1,300

[1] Includes straight-line rent adjustments of approximately $4.8 million and $5.1 million for the three months ended September 30, 2018 and 2017, respectively, and $9.8 million and $16.4 million for the nine months ended September 30, 2018 and 2017

[2] During the three months ended September 30, 2018, the joint venture that owns Metropolitan Square, in Washington, DC, commenced a renovation project and recorded accelerated depreciation of approximately $22.4 million related to the remaining book value of the asset to be replaced. The Company's share of the accelerated depreciation totaled approximately $4.5 million.

[3] Includes straight-line rent adjustments of approximately $0.5 million and $0.7 million for the three months ended September 30, 2018 and 2017, respectively, and $1.9 million and $2.2 million for the nine months ended September 30, 2018 and 2017, respectively. Also includes net above-/below-market rent adjustments of approximately $0.4 million and $0.4 million for the three months ended September 30, 2018 and 2017, respectively, and $1.2 million and $1.3 million for the nine months ended September 30, 2018 and 2017