Investments in Unconsolidated Joint Ventures - Acquisition, Development and Construction Arrangements/Sale of Joint Venture Interest or Property (Details) - USD ($) $ in Thousands	1 Months Ended					3 Months Ended		6 Months Ended
	Jun. 30, 2018	May 31, 2018	Apr. 30, 2018	Mar. 31, 2018	Feb. 28, 2018	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Oct. 31, 2016
Schedule of Equity Method Investments [Line Items]
Equity method investments	$ 3,059,985					$ 3,059,985		$ 3,059,985		$ 2,362,989
Equity in net gain on sale of interest in unconsolidated joint venture/real estate						72,025	$ 13,089	65,585	$ 15,136
Stonehenge Portfolio
Schedule of Equity Method Investments [Line Items]
Ownership Interest											90.00%
Preferred Equity due September 2018
Schedule of Equity Method Investments [Line Items]
Equity method investments	141,000					141,000		141,000		0
Mezzanine loan due February 2022
Schedule of Equity Method Investments [Line Items]
Equity method investments	44,483					44,483		44,483		44,823
Mezzanine loan due July 2036
Schedule of Equity Method Investments [Line Items]
Equity method investments	28,105					28,105		28,105		26,716
Mezzanine Loan and Preferred Equity, Due March 2018
Schedule of Equity Method Investments [Line Items]
Equity method investments	0					0		0		100,000
Mezzanine Loans And Preferred Equity [Member]
Schedule of Equity Method Investments [Line Items]
Equity method investments	$ 213,588					$ 213,588		$ 213,588		$ 171,539
1274 Fifth Avenue
Schedule of Equity Method Investments [Line Items]
Ownership Interest					9.83%
Gross Asset Valuation					$ 44,100
Equity in net gain on sale of interest in unconsolidated joint venture/real estate					$ (362)
1515 Broadway
Schedule of Equity Method Investments [Line Items]
Ownership Interest					13.00%
Gross Asset Valuation					$ 1,950,000
Equity in net gain on sale of interest in unconsolidated joint venture/real estate					$ 0
Stonehenge Portfolio
Schedule of Equity Method Investments [Line Items]
Ownership Interest				5.00%
Gross Asset Valuation				$ 287,000
Equity in net gain on sale of interest in unconsolidated joint venture/real estate				$ (5,701)
175-225 Third Street Brooklyn, New York
Schedule of Equity Method Investments [Line Items]
Ownership Interest			95.00%
Gross Asset Valuation			$ 115,000
Equity in net gain on sale of interest in unconsolidated joint venture/real estate			$ 19,483
1745 Broadway
Schedule of Equity Method Investments [Line Items]
Ownership Interest		56.87%
Gross Asset Valuation		$ 633,000
Equity in net gain on sale of interest in unconsolidated joint venture/real estate		$ 52,038
Jericho Plaza
Schedule of Equity Method Investments [Line Items]
Ownership Interest	11.67%					11.67%		11.67%
Gross Asset Valuation	$ 117,400					$ 117,400		$ 117,400
Equity in net gain on sale of interest in unconsolidated joint venture/real estate	$ 147