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								9 Months Ended	12 Months Ended
	Nov. 30, 2018 USD ($)	Jul. 31, 2018 USD ($)	Jun. 30, 2018 USD ($)	May 31, 2018 USD ($)	Apr. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Feb. 28, 2018 USD ($)	May 31, 2017 USD ($)	Sep. 30, 2016 USD ($)	May 31, 2016 USD ($)	Mar. 31, 2016 USD ($)	Feb. 29, 2016 USD ($)	Dec. 31, 2018 USD ($) ft² note	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2018 USD ($) ft² note	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Aug. 31, 2018 USD ($)	Apr. 30, 2017 USD ($) ft²	Dec. 31, 2015
Schedule of Equity Method Investments [Line Items]
Investments in unconsolidated joint ventures													$ 3,019,020				$ 2,362,989					$ 3,019,020	$ 2,362,989
Acquisition, development and construction arrangements, carrying value													44,357				171,539					44,357	171,539
Gain (Loss) on Sale													167,445	$ 70,937	$ 72,025	$ (6,440)	0	$ 1,030	$ 13,089	$ 2,047		303,967	16,166	$ 44,009
Employee-related liabilities													$ 11,700				0					$ 11,700	0	1,100
Approximate Square Feet | ft²													30,177,253									30,177,253
Gain on sale of real estate, net													$ (36,984)	$ (2,504)	$ (14,790)	$ 23,521	76,497	$ 0	$ (3,823)	$ 567		$ (30,757)	73,241	$ 238,116
Participating Financing Due February 2022
Schedule of Equity Method Investments [Line Items]
Investments in unconsolidated joint ventures													44,357				44,823					44,357	44,823
Mezzanine Loan And Preferred Equity, Due March 2018
Schedule of Equity Method Investments [Line Items]
Investments in unconsolidated joint ventures													0				100,000					0	100,000
Mezzanine Loan Due July 2036
Schedule of Equity Method Investments [Line Items]
Investments in unconsolidated joint ventures													0				$ 26,716					0	$ 26,716
747 Madison Avenue
Schedule of Equity Method Investments [Line Items]
Gain on sale of real estate, net								$ 13,000
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
Mezzanine Loan
Schedule of Equity Method Investments [Line Items]
Ownership Interest Sold																									33.33%
Gross asset valuation																									$ 15,000
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
Jericho Plaza
Schedule of Equity Method Investments [Line Items]
Ownership Interest Sold			11.67%								11.67%				11.67%												77.78%
Gross asset valuation			$ 117,400												$ 117,400
Gain (Loss) on Sale			$ 147
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
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
Stonehenge Portfolio
Schedule of Equity Method Investments [Line Items]
Ownership Interest Sold						0.50%										0.50%
Gross asset valuation						$ 287,000										$ 287,000
Gain (Loss) on Sale						$ (5,701)
1515 Broadway
Schedule of Equity Method Investments [Line Items]
Ownership Interest Sold							13.00%
Gross asset valuation							$ 1,950,000						$ 1,950,000									$ 1,950,000
Gain (Loss) on Sale							$ 0
Approximate Square Feet | ft²													1,750,000									1,750,000
1274 Fifth Avenue
Schedule of Equity Method Investments [Line Items]
Ownership Interest Sold							9.83%
Gross asset valuation							$ 44,100
Gain (Loss) on Sale							$ (362)
102 Greene Street
Schedule of Equity Method Investments [Line Items]
Ownership Interest Sold																		10.00%			10.00%
Gross asset valuation																		$ 43,500			$ 43,500					$ 43,500
Gain (Loss) on Sale																					$ 283
Employee-related liabilities																										$ 900
Approximate Square Feet | ft²																										9,200
76 11th Avenue
Schedule of Equity Method Investments [Line Items]
Ownership Interest Sold								33.33%
Gross asset valuation								$ 138,240
Number of notes owned | note													2									2
Stonehenge Portfolio
Schedule of Equity Method Investments [Line Items]
Gross asset valuation								300,000
Gain (Loss) on Sale								$ 871
EOP Denver
Schedule of Equity Method Investments [Line Items]
Ownership Interest Sold																		0.48%			0.48%
Gross asset valuation																		$ 180,700			$ 180,700
Gain (Loss) on Sale									$ 300
33 Beekman
Schedule of Equity Method Investments [Line Items]
Ownership Interest Sold										45.90%
Gross asset valuation										$ 196,000
Gain (Loss) on Sale										33,000
Promote recognized										$ 10,800
EOP Denver
Schedule of Equity Method Investments [Line Items]
Ownership Interest Sold																				4.79%
Gross asset valuation																				$ 180,700
Gain (Loss) on Sale											$ 2,800
7 Renaissance
Schedule of Equity Method Investments [Line Items]
Ownership Interest Sold																				50.00%
Gross asset valuation																				$ 20,700
Gain (Loss) on Sale											$ 4,200
1 Jericho
Schedule of Equity Method Investments [Line Items]
Ownership Interest Sold												66.11%
Gross asset valuation												$ 95,200
Gain (Loss) on Sale												$ 3,300
Joint venture
Schedule of Equity Method Investments [Line Items]
Investments in unconsolidated joint ventures													$ 3,019,020									$ 3,019,020