Investments in Unconsolidated Joint Ventures (Statements of Operations of the Joint Ventures) (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Schedule of Equity Method Investments [Line Items]
Total revenue		$ 655,229	$ 657,712	$ 656,907	$ 632,228	$ 636,061	$ 625,228	$ 623,546	$ 665,985	$ 2,602,076		$ 2,550,820		$ 2,490,821
Expenses
Depreciation and amortization										617,547		694,403		639,542
Total expenses										1,693,966		1,725,036		1,641,456
Operating income										908,110		825,784		849,365
Other income (expense)
Interest expense										(374,481)		(412,849)		(432,196)
Net income										562,481		569,977		799,918
Income from unconsolidated joint ventures										11,232		8,074		22,770
Gain on sale of investment in unconsolidated joint venture										0		59,370		0
Straight-line rent adjustments										54,800		31,700		80,000
Mortgage and notes payable, net		2,979,281				2,063,087				2,979,281		2,063,087
Above and below market rent adjustments, net										23,500		30,200		35,900
Gain on sale of investment in unconsolidated joint venture										0		55,707		0
Unconsolidated Joint Ventures [Member]
Schedule of Equity Method Investments [Line Items]
Total revenue	[1]									222,517		177,182		155,642
Expenses
Operating										90,542		76,741		65,093
Depreciation and amortization										57,079		44,989		36,057
Total expenses										147,621		121,730		101,150
Operating income										74,896		55,452		54,492
Other income (expense)
Interest expense										(46,371)		(34,016)		(32,176)
Net income										28,525		21,436		22,316
Company's share of net income										18,439		9,873		22,031	[2]
Basis differential										(7,207)	[3]	(1,799)	[3]	739
Income from unconsolidated joint ventures										11,232		8,074		22,770
Straight-line rent adjustments										21,700		18,100		3,900
Mortgage and notes payable, net		$ 1,437,440				$ 865,665				1,437,440		865,665
BP/CRF 901 New York Avenue LLC [Member]
Other income (expense)
Mortgage and notes payable, net														225,000
Gain on sale of investment in unconsolidated joint venture														$ 24,500
Colorado Center [Member]
Other income (expense)
Straight-line rent adjustments										1,900		1,400
Above and below market rent adjustments, net										$ 2,900		$ 900

[1] Includes straight-line rent adjustments of approximately $21.7 million, $18.1 million and $3.9 million for the years ended December 31, 2017, 2016 and 2015, respectively.

[2] During the year ended December 31, 2015, the Company received a distribution of approximately $24.5 million, which was generated from the excess loan proceeds from the refinancing of 901 New York Avenue’s mortgage loan to a new 10-year mortgage loan totaling $225.0 million. The Company’s allocation of income and distributions for the year ended December 31, 2015 was not proportionate to its nominal ownership interest as a result of the achievement of specified investment return thresholds, as provided for in the joint venture agreement.

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