Investments in Unconsolidated Joint Ventures - Acquisition, Development and Construction Arrangements/Sale of Joint Venture Interest or Property (Details) $ in Thousands	1 Months Ended											3 Months Ended								12 Months Ended
	Dec. 31, 2019 USD ($) note	May 31, 2019 USD ($)	Jan. 31, 2019 USD ($)	Nov. 30, 2018 USD ($)	Jul. 31, 2018 USD ($)	Jun. 30, 2018 USD ($)	May 31, 2018 USD ($)	Apr. 30, 2018 USD ($)	Feb. 28, 2018 USD ($)	Sep. 30, 2017 USD ($)	May 31, 2017 USD ($)	Dec. 31, 2019 USD ($) note	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2019 USD ($) note	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Aug. 31, 2018 USD ($)	Apr. 30, 2017 USD ($)
Schedule of Equity Method Investments [Line Items]
Investments in unconsolidated joint ventures	$ 2,912,842											$ 2,912,842				$ 3,019,020				$ 2,912,842	$ 3,019,020
Gain (Loss) on Sale												0	$ 0	$ 59,015	$ 17,166	167,445	$ 70,937	$ 72,025	$ (6,440)	76,181	303,967	$ 16,166
Employee-related liabilities	4,000											4,000				11,700				4,000	11,700	0
(Loss) gain on sale of real estate, net												(19,241)	$ 3,541	$ 0	$ (1,049)	(36,984)	$ (2,504)	$ (14,790)	$ 23,521	(16,749)	(30,757)	73,241
Madison Avenue 747
Schedule of Equity Method Investments [Line Items]
(Loss) gain on sale of real estate, net											$ 13,000
Participating Financing Due February 2022
Schedule of Equity Method Investments [Line Items]
Investments in unconsolidated joint ventures	0											0				44,357				0	44,357
Mezzanine Loans And Preferred Equity
Schedule of Equity Method Investments [Line Items]
Acquisition, development and construction arrangements, carrying value	$ 0											$ 0				44,357				$ 0	44,357
1515 Broadway
Schedule of Equity Method Investments [Line Items]
Gross asset valuation																						1,950,000
102 Greene Street
Schedule of Equity Method Investments [Line Items]
Gross asset valuation																								$ 43,500
Employee-related liabilities																								$ 900
76 11th Avenue
Schedule of Equity Method Investments [Line Items]
Number of notes owned | note	2											2								2
Joint venture
Schedule of Equity Method Investments [Line Items]
Investments in unconsolidated joint ventures	$ 2,912,842											$ 2,912,842								$ 2,912,842
Joint venture | East 21 Street 66
Schedule of Equity Method Investments [Line Items]
Gross asset valuation	2,900											2,900								2,900
Gain (Loss) on Sale	279
Joint venture | Fifth Avenue, 521
Schedule of Equity Method Investments [Line Items]
Ownership Interest Sold		50.50%
Gross asset valuation		$ 381,000
Gain (Loss) on Sale		$ 57,874
Joint venture | Spring Street 131 to 137
Schedule of Equity Method Investments [Line Items]
Ownership Interest Sold			20.00%
Gross asset valuation			$ 216,000
Gain (Loss) on Sale			$ 17,660
Joint venture | Stonehenge Portfolio
Schedule of Equity Method Investments [Line Items]
Gross asset valuation	$ 468,800											$ 468,800				$ 331,100				468,800	331,100	300,000
Gain (Loss) on Sale																				$ (2,408)	$ (6,063)	$ 871
Joint venture | 3 Columbus Circle
Schedule of Equity Method Investments [Line Items]
Ownership Interest Sold				48.90%
Gross asset valuation				$ 851,000
Gain (Loss) on Sale				$ 160,368
Joint venture | Mezzanine Loan
Schedule of Equity Method Investments [Line Items]
Ownership Interest Sold																							33.33%
Gross asset valuation																							$ 15,000
Joint venture | 724 Fifth Avenue
Schedule of Equity Method Investments [Line Items]
Ownership Interest Sold					49.90%
Gross asset valuation					$ 365,000
Gain (Loss) on Sale					$ 64,587
Joint venture | Jericho Plaza
Schedule of Equity Method Investments [Line Items]
Ownership Interest Sold						11.67%												11.67%
Gross asset valuation						$ 117,400												$ 117,400
Gain (Loss) on Sale						$ 147
Joint venture | 1745 Broadway
Schedule of Equity Method Investments [Line Items]
Ownership Interest Sold							56.87%
Gross asset valuation							$ 633,000
Gain (Loss) on Sale							$ 52,038
Joint venture | 175-225 Third Street Brooklyn, New York
Schedule of Equity Method Investments [Line Items]
Ownership Interest Sold								95.00%
Gross asset valuation								$ 115,000
Gain (Loss) on Sale								$ 19,483
Joint venture | 1515 Broadway
Schedule of Equity Method Investments [Line Items]
Ownership Interest Sold									13.00%
Gross asset valuation									$ 1,950,000
Gain (Loss) on Sale									$ 0
Joint venture | 102 Greene Street
Schedule of Equity Method Investments [Line Items]
Ownership Interest Sold										10.00%
Gross asset valuation										$ 43,500
Gain (Loss) on Sale										$ 283
Joint venture | 76 11th Avenue
Schedule of Equity Method Investments [Line Items]
Ownership Interest Sold											33.33%
Gross asset valuation											$ 138,240