Investments in Partially Owned Entities (85 Tenth Avenue, Fairfax Square) (Details) ft² in Thousands		1 Months Ended			12 Months Ended
	Dec. 02, 2016 USD ($)	Dec. 19, 2016 USD ($)	Aug. 31, 2014 USD ($)	Feb. 28, 2013 USD ($)	Dec. 31, 2016 USD ($) ft²	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2007 USD ($)
Schedule Of Equity Method Investments
Payments To Acquire Equity Method Investments					$ 127,608,000	$ 235,439,000	$ 120,639,000
Principal amount of mortgage loans					$ 9,278,263,000	9,513,713,000
85 Tenth Avenue | Junior and Senior Mezzanine Loans | Office
Schedule Of Equity Method Investments
Square Footage Of Real Estate Property Sold | ft²					626
Percent of Equity Interest Allow to Acquire								49.90%
Payments To Acquire Equity Method Investments								$ 50,000,000
Fairfax Square
Schedule Of Equity Method Investments
Proceeds from the sale of Joint Venture		$ 15,500,000
Net realized gain on sale of investment		$ 15,302,000
Vornado Realty Trust | 85 Tenth Avenue | Office
Schedule Of Equity Method Investments
Net proceeds from loan investments	$ 191,779,000
Joint Venture | 85 Tenth Avenue
Schedule Of Equity Method Investments
Equity method ownership percentage					49.90%
Interest Rate, End of Period (in percentage)					4.55%
Income from the repayment of loans and preferred equity					$ 160,843,000	$ 0	$ 0
Joint Venture | 85 Tenth Avenue | Office
Schedule Of Equity Method Investments
Equity method ownership percentage	49.90%
Payments To Acquire Equity Method Investments			$ 8,413,000
Total equity mehtod investment			$ 72,996,000
Joint Venture | Vornado Realty Trust | 85 Tenth Avenue | Office
Schedule Of Equity Method Investments
Investment in loans and equity reduced amount	$ 30,936,000
Joint Venture | Vornado Realty Trust | 85 Tenth Avenue | Senior Mezzanine loans | Office
Schedule Of Equity Method Investments
Payments To Acquire Equity Method Investments				$ 14,583,000
Equity method investee | 85 Tenth Avenue | Office
Schedule Of Equity Method Investments
Debt Term	10 years				10 years
Mortgage loan interest rate	4.55%
Principal amount of mortgage loans	$ 625,000,000
Co Venture | Fairfax Square
Schedule Of Equity Method Investments
Ownership sold, percentage		20.00%