Investments in Unconsolidated Joint Ventures - Acquisition, Development and Construction Arrangements/Sale of Joint Venture Interest or Property (Details) - USD ($) $ in Thousands	1 Months Ended										12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	May 31, 2019	Jan. 31, 2019	Nov. 30, 2018	Jul. 31, 2018	Jun. 30, 2018	May 31, 2018	Apr. 30, 2018	Feb. 28, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Jun. 30, 2019	Aug. 31, 2018
Schedule of Equity Method Investments [Line Items]
Gain (Loss) on Sale											$ 2,961	$ 76,181	$ 303,967
Employee-related liabilities	$ 0	$ 4,000									$ 0	4,000	11,700
Joint venture | 333 East 22nd St
Schedule of Equity Method Investments [Line Items]
Ownership Interest Sold	33.33%										33.33%
Gross asset valuation	$ 1,640										$ 1,640
Gain (Loss) on Sale	$ 2,968
Joint venture | East 21 Street 66
Schedule of Equity Method Investments [Line Items]
Gross asset valuation		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
Gain (Loss) on Sale												$ (2,408)	$ (6,063)
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%
Gross asset valuation														$ 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