Investments in Unconsolidated Joint Ventures - Acquisition, Development and Construction Arrangements/Sale of Joint Venture Interest or Property (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	May 31, 2017 note	Dec. 31, 2016 USD ($)	Oct. 31, 2016
Schedule of Equity Method Investments [Line Items]
Acquisition, development and construction arrangements, carrying value	$ 170,735		$ 170,735			$ 170,164
Gain (loss) on sale of real estate, net	0	$ 397	(3,256)	$ 210,750
Mezzanine Loan and Preferred Equity Due March 2018
Schedule of Equity Method Investments [Line Items]
Acquisition, development and construction arrangements, carrying value	100,000		100,000			100,000
Mezzanine loan due February 2022
Schedule of Equity Method Investments [Line Items]
Acquisition, development and construction arrangements, carrying value	44,881		44,881			45,622
Mezzanine loan due July 2036
Schedule of Equity Method Investments [Line Items]
Acquisition, development and construction arrangements, carrying value	25,854		25,854			$ 24,542
Stonehenge Portfolio
Schedule of Equity Method Investments [Line Items]
Ownership Interest							90.00%
Gross Asset Valuation	$ 300,000		300,000
Gain on Sale			$ 871
102 Greene Street
Schedule of Equity Method Investments [Line Items]
Ownership Interest	10.00%		10.00%
Gross Asset Valuation	$ 43,500		$ 43,500
Gain on Sale			$ 283
747 Madison Avenue
Schedule of Equity Method Investments [Line Items]
Gain (loss) on sale of real estate, net	$ 13,000
76 11th Avenue
Schedule of Equity Method Investments [Line Items]
Number of notes owned by joint venture partner | note					2