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		9 Months Ended
	May 31, 2019	Feb. 28, 2018	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Oct. 31, 2016
Schedule of Equity Method Investments [Line Items]
Equity method investments			$ 2,923,595		$ 2,923,595		$ 3,019,020
Equity in net (loss) gain on sale of interest in unconsolidated joint venture/real estate			0	$ 70,937	76,181	$ 136,522
Stonehenge Portfolio
Schedule of Equity Method Investments [Line Items]
Ownership Interest								90.00%
Mezzanine loan due February 2022
Schedule of Equity Method Investments [Line Items]
Equity method investments			0		0		44,357
Mezzanine Loans And Preferred Equity
Schedule of Equity Method Investments [Line Items]
Equity method investments			0		0		$ 44,357
131-137 Spring Street
Schedule of Equity Method Investments [Line Items]
Ownership Interest		20.00%
Gross Asset Valuation		$ 216,000
Equity in net (loss) gain on sale of interest in unconsolidated joint venture/real estate		$ 17,660
521 Fifth Avenue
Schedule of Equity Method Investments [Line Items]
Ownership Interest	50.50%
Gross Asset Valuation	$ 381,000
Equity in net (loss) gain on sale of interest in unconsolidated joint venture/real estate	$ 57,874
Stonehenge Portfolio
Schedule of Equity Method Investments [Line Items]
Gross Asset Valuation			$ 468,800		468,800
Equity in net (loss) gain on sale of interest in unconsolidated joint venture/real estate					$ (2,408)