Investments in Unconsolidated Joint Ventures (Statements of Operations of the Joint Ventures) (Details) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($) yr		Jun. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)
Schedule of Equity Method Investments [Line Items]
Total revenue		$ 618,221	$ 589,794	$ 1,236,697		$ 1,164,488
Expenses
Depreciation and amortization		167,844	154,628	322,067		308,898
Total expenses		413,295	388,521	825,562		786,318
Operating income		204,926	201,273	411,135		378,170
Other expense
Interest expense		108,534	110,977	217,291		224,531
Net income		100,739	95,901	309,909		163,657
Income from unconsolidated joint ventures		3,078	2,834	17,912		5,650
Distribution related to excess loan proceeds				24,527		357
Mortgage loan		4,269,808		4,269,808			$ 4,309,484
Unconsolidated Joint Ventures [Member]
Schedule of Equity Method Investments [Line Items]
Total revenue	[1]	39,152	38,437	78,684		76,471
Expenses
Operating		15,824	15,461	32,099		30,925
Depreciation and amortization		8,951	9,167	18,022		18,259
Total expenses		24,775	24,628	50,121		49,184
Operating income		14,377	13,809	28,563		27,287
Other expense
Interest expense		7,986	7,984	15,966		15,996
Net income		6,391	5,825	12,597		11,291
Company's share of net income (loss)		2,902	2,578	17,544	[2]	5,203
Basis differential		176	256	368		447
Income from unconsolidated joint ventures		3,078	2,834	17,912		5,650
Straight-line rent adjustments		300	300	2,000		900
"Above" and "below" market rent adjustments, net		(100)	$ (100)	(100)		$ 0
Mortgage loan		832,860		832,860			$ 830,075
901 New York Avenue Llc [Member]
Other expense
Distribution related to excess loan proceeds				24,500
Mortgage loan		$ 225,000		$ 225,000
Mortgage loan term (in years) | yr				10

[1] Includes straight-line rent adjustments of approximately $0.3 million and $0.3 million for the three months ended June 30, 2015 and 2014, respectively, and approximately $2.0 million and $0.9 million for the six months ended June 30, 2015 and 2014. Includes net above-/below-market rent adjustments of approximately $(0.1) million and $(0.1) million for the three months ended June 30, 2015 and 2014, respectively, and approximately $(0.1) million and $0.0 million for the six months ended June 30, 2015 and 2014, respectively.

[2] During the six months ended June 30, 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 six months ended June 30, 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.