Investments in Unconsolidated Joint Ventures (Acquisition, Development and Construction Arrangements/Sale of Joint Venture Interest or Property)(Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	May 31, 2017 note	Dec. 31, 2016 USD ($)
Schedule of Equity Method Investments [Line Items]
Acquisition, development and construction arrangements, carrying value	$ 170,342		$ 170,342			$ 170,164
(Loss) gain on sale of real estate, net	(3,823)	$ 196,580	(3,256)	$ 210,353
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	44,939		44,939			45,622
Mezzanine loan due July 2036
Schedule of Equity Method Investments [Line Items]
Acquisition, development and construction arrangements, carrying value	25,403		25,403			$ 24,542
Stonehenge Portfolio
Schedule of Equity Method Investments [Line Items]
Gross Asset Valuation	300,000		300,000
Gain on Sale			$ 871
747 Madison Avenue
Schedule of Equity Method Investments [Line Items]
(Loss) gain 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