Investments in Unconsolidated Joint Ventures (Statements of Operations of the Joint Ventures) (Details) (USD $)	3 Months Ended
	Mar. 31, 2015		Mar. 31, 2014		Dec. 31, 2014
Schedule of Equity Method Investments [Line Items]
Total revenue	$ 618,476,000		$ 574,694,000
Expenses
Depreciation and amortization	154,223,000		154,270,000
Total expenses	412,267,000		397,797,000
Operating income	206,209,000		176,897,000
Other expense
Interest expense	108,757,000		113,554,000
Net income	209,170,000		67,756,000
Income from unconsolidated joint ventures	14,834,000		2,816,000
Distribution related to excess loan proceeds	24,527,000		113,000
Mortgage loan	4,289,120,000				4,309,484,000
Unconsolidated Joint Ventures [Member]
Schedule of Equity Method Investments [Line Items]
Total revenue	39,532,000	[1]	38,034,000	[1]
Expenses
Operating	16,275,000		15,464,000
Depreciation and amortization	9,071,000		9,092,000
Total expenses	25,346,000		24,556,000
Operating income	14,186,000		13,478,000
Other expense
Interest expense	7,980,000		8,012,000
Net income	6,206,000		5,466,000
Company's share of net income (loss)	14,642,000	[2]	2,625,000
Basis differential	192,000		191,000
Income from unconsolidated joint ventures	14,834,000		2,816,000
Straight-line rent adjustments	1,600,000		600,000
"Above" and "below" market rent adjustments, net	(100,000)		100,000
Mortgage loan	831,039,000				830,075,000
901 New York Avenue Llc [Member]
Other expense
Distribution related to excess loan proceeds	24,500,000
Mortgage loan	$ 225,000,000
Mortgage loan term (in years)	10

[1] Includes straight-line rent adjustments of $1.6 million and $0.6 million for the three months ended March 31, 2015 and 2014, respectively. Includes net above-/below-market rent adjustments of $(0.1) million and $0.1 million for the three months ended March 31, 2015 and 2014, respectively.

[2] (2)During the three months ended March 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 three months ended March 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.