Investment in Unconsolidated Joint Ventures Investment in Unconsolidated Joint Ventures-Additional Information (Details) (USD $)	3 Months Ended		6 Months Ended			3 Months Ended		6 Months Ended			0 Months Ended				1 Months Ended		1 Months Ended
	Jun. 30, 2014 sqft	Jun. 30, 2013	Jun. 30, 2014 sqft	Jun. 30, 2013	Dec. 31, 2013	Jun. 30, 2014 Joint Venture	Jun. 30, 2013 Joint Venture	Jun. 30, 2014 Joint Venture	Jun. 30, 2013 Joint Venture	Dec. 31, 2013 Joint Venture	May 31, 2014 747 Madison Avenue	May 31, 2014 747 Madison Avenue unit	Jun. 30, 2014 747 Madison Avenue Joint Venture	May 31, 2014 747 Madison Avenue Joint Venture	Mar. 31, 2014 West Coast office portfolio	Mar. 31, 2014 West Coast office portfolio Joint Venture	Jan. 31, 2014 21 West 34th Street sqft	Jan. 31, 2014 21 West 34th Street Joint Venture	Mar. 31, 2014 Participating Financing Due March 2016
Investment in Unconsolidated Joint Ventures
Number of units in property (units)												5
Real Estate fair value												$ 160,000,000			$ 756,000,000		$ 114,900,000
Total floating rate debt	1,508,351,000		1,508,351,000		729,949,000	1,682,685,000		1,682,685,000		1,842,815,000				33,100,000		526,300,000		100,000,000
Ownership Interest (as a percent)													100.00%
Joint venture ownership percentage sold (percent)											33.33%				43.74%		49.90%
Promote income											10,300,000
Preferred equity investment												30,000,000
Deferred gain on sale											13,100,000
Equity in net gain (loss) on sale of interest in unconsolidated joint venture/real estate	1,444,000	(3,583,000)	106,084,000	(3,583,000)											85,500,000		20,900,000
Loans Receivable, Gross, Commercial, Acquisition																			100,000,000
Loans and Leases Receivable, Gross																			60,000,000
Debt and preferred equity investments	1,547,808,000		1,547,808,000		1,304,839,000														40,000,000
Management fees, base revenue						$ 4,800,000	$ 1,400,000	$ 11,200,000	$ 4,000,000
Area of property (sqft)	33,867,788		33,867,788														91,311