Investments in Unconsolidated Joint Ventures - Mortgages and Other Loans Payable (Details)	9 Months Ended
	Sep. 30, 2017 USD ($) extenstion_option	Oct. 31, 2017 USD ($)	Aug. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Debt Instrument [Line Items]
Total fixed rate debt	$ 3,286,248,000			$ 3,518,127,000
Total floating rate debt	558,813,000			622,585,000
Total fixed rate and floating rate debt	$ 3,845,061,000			4,140,712,000
Number of extension options | extenstion_option	2
Period of extension options	1 year
521 Fifth Avenue
Debt Instrument [Line Items]
Economic Interest (as a percent)	50.50%
717 Fifth Avenue
Debt Instrument [Line Items]
Economic Interest (as a percent)	10.92%
650 Fifth Avenue
Debt Instrument [Line Items]
Economic Interest (as a percent)	50.00%
21 East 66th Street
Debt Instrument [Line Items]
Economic Interest (as a percent)	32.28%
3 Columbus Circle
Debt Instrument [Line Items]
Economic Interest (as a percent)	48.90%
11 Madison Avenue
Debt Instrument [Line Items]
Economic Interest (as a percent)	60.00%
800 Third Avenue
Debt Instrument [Line Items]
Economic Interest (as a percent)	60.52%
East 400 Street 57
Debt Instrument [Line Items]
Economic Interest (as a percent)	41.00%
1745 Broadway
Debt Instrument [Line Items]
Economic Interest (as a percent)	56.87%
55 West 46th Street
Debt Instrument [Line Items]
Economic Interest (as a percent)	25.00%
175-225 Third Street Brooklyn, New York
Debt Instrument [Line Items]
Economic Interest (as a percent)	95.00%
Jericho Plaza
Debt Instrument [Line Items]
Economic Interest (as a percent)	11.67%
724 Fifth Avenue
Debt Instrument [Line Items]
Economic Interest (as a percent)	50.00%
1552 Broadway
Debt Instrument [Line Items]
Economic Interest (as a percent)	50.00%
280 Park Avenue (9)
Debt Instrument [Line Items]
Economic Interest (as a percent)	50.00%
121 Greene Street
Debt Instrument [Line Items]
Economic Interest (as a percent)	50.00%
10 East 53rd Street
Debt Instrument [Line Items]
Economic Interest (as a percent)	55.00%
131-137 Spring Street
Debt Instrument [Line Items]
Economic Interest (as a percent)	20.00%
11 West 34th Street
Debt Instrument [Line Items]
Economic Interest (as a percent)	30.00%
100 Park Avenue
Debt Instrument [Line Items]
Economic Interest (as a percent)	49.90%
One Vanderbilt
Debt Instrument [Line Items]
Economic Interest (as a percent)	71.01%
Total floating rate debt	$ 0			64,030,000
One Vanderbilt | Construction Loans
Debt Instrument [Line Items]
Term (in Years)	5 years
Maximum facility capacity	$ 1,500,000,000.0
Credit facility, interest rate (as a percent)	3.50%
Number of extension options | extenstion_option	2
Period of extension options	1 year
605 West 42nd Street
Debt Instrument [Line Items]
Economic Interest (as a percent)	20.00%
Joint venture
Debt Instrument [Line Items]
Total fixed rate debt	$ 3,448,423,000			3,566,846,000
Total floating rate debt	3,877,504,000			2,982,528,000
Total fixed rate and floating rate debt	7,325,927,000			6,549,374,000
Deferred financing costs, net	(127,318,000)			(95,408,000)
Total joint venture mortgages and other loans payable, net	7,198,609,000			6,453,966,000
Joint venture | 521 Fifth Avenue
Debt Instrument [Line Items]
Total fixed rate debt	170,000,000			170,000,000
Joint venture | 717 Fifth Avenue | Mortgage loan
Debt Instrument [Line Items]
Committed amount	300,000,000
Joint venture | 717 Fifth Avenue | Mezzanine loans
Debt Instrument [Line Items]
Committed amount	355,300,000
Joint venture | 717 Fifth Avenue | Initial Maturity July 2022
Debt Instrument [Line Items]
Total fixed rate debt	300,000,000			300,000,000
Joint venture | 717 Fifth Avenue | Initial Maturity July 2022, 2
Debt Instrument [Line Items]
Total fixed rate debt	355,328,000			355,328,000
Joint venture | 650 Fifth Avenue
Debt Instrument [Line Items]
Total floating rate debt	0			77,500,000
Joint venture | 650 Fifth Avenue | Initial Maturity October 2022
Debt Instrument [Line Items]
Total fixed rate debt	225,000,000			0
Joint venture | 21 East 66th Street
Debt Instrument [Line Items]
Total fixed rate debt	12,000,000			12,000,000
Total floating rate debt	1,667,000			1,726,000
Joint venture | 3 Columbus Circle
Debt Instrument [Line Items]
Total fixed rate debt	350,000,000			350,000,000
Joint venture | 11 Madison Avenue
Debt Instrument [Line Items]
Total fixed rate debt	1,400,000,000			1,400,000,000
Joint venture | 800 Third Avenue
Debt Instrument [Line Items]
Total fixed rate debt	177,000,000			177,000,000
Joint venture | East 400 Street 57
Debt Instrument [Line Items]
Total fixed rate debt	100,000,000			100,000,000
Joint venture | Stonehenge Portfolio
Debt Instrument [Line Items]
Total fixed rate debt	359,095,000			362,518,000
Total floating rate debt	55,340,000			65,577,000
Joint venture | Stonehenge Portfolio | Initial Maturity November 2017
Debt Instrument [Line Items]
Total fixed rate debt	34,000,000
Joint venture | Stonehenge Portfolio | Initial Maturity August 2019
Debt Instrument [Line Items]
Total fixed rate debt	137,700,000
Joint venture | Stonehenge Portfolio | Initial Maturity June 2024
Debt Instrument [Line Items]
Total fixed rate debt	172,500,000
Joint venture | Stonehenge Portfolio | Initial Maturity February 2027
Debt Instrument [Line Items]
Total fixed rate debt	14,900,000
Joint venture | 1745 Broadway
Debt Instrument [Line Items]
Total fixed rate debt	0			340,000,000
Total floating rate debt	345,000,000			0
Joint venture | 1745 Broadway | Mortgage loan
Debt Instrument [Line Items]
Committed amount	375,000,000
Unfunded amount	30,000,000
Joint venture | 55 West 46th Street
Debt Instrument [Line Items]
Total floating rate debt	165,328,000			157,322,000
Joint venture | 55 West 46th Street | Mortgage loan
Debt Instrument [Line Items]
Committed amount	190,000,000
Unfunded amount	24,700,000
Joint venture | 175-225 Third Street Brooklyn, New York
Debt Instrument [Line Items]
Total floating rate debt	40,000,000			40,000,000
Joint venture | Jericho Plaza
Debt Instrument [Line Items]
Total floating rate debt	$ 79,530,000			76,993,000
Term (in Years)	2 years
Face amount of loan	$ 100,000,000
Joint venture | 724 Fifth Avenue
Debt Instrument [Line Items]
Total floating rate debt	275,000,000			275,000,000
Joint venture | 1552 Broadway
Debt Instrument [Line Items]
Total floating rate debt	185,410,000			185,410,000
Joint venture | 1552 Broadway | Mortgage loan
Debt Instrument [Line Items]
Committed amount	145,000,000
Unfunded amount	600,000
Joint venture | 1552 Broadway | Mezzanine loans
Debt Instrument [Line Items]
Committed amount	41,500,000
Unfunded amount	500,000
Joint venture | 280 Park Avenue (9)
Debt Instrument [Line Items]
Total floating rate debt	1,200,000,000			900,000,000
Joint venture | 280 Park Avenue (9) | Mortgage loan
Debt Instrument [Line Items]
Committed amount			$ 1,075,000,000
Joint venture | 280 Park Avenue (9) | Mezzanine loans
Debt Instrument [Line Items]
Committed amount			125,000,000
Joint venture | 121 Greene Street
Debt Instrument [Line Items]
Total floating rate debt	15,000,000			15,000,000
Joint venture | 10 East 53rd Street
Debt Instrument [Line Items]
Total floating rate debt	170,000,000			125,000,000
Joint venture | 131-137 Spring Street
Debt Instrument [Line Items]
Total floating rate debt	141,000,000			141,000,000
Joint venture | 11 West 34th Street
Debt Instrument [Line Items]
Total floating rate debt	23,000,000			23,000,000
Joint venture | 100 Park Avenue
Debt Instrument [Line Items]
Total floating rate debt	360,000,000			360,000,000
Joint venture | One Vanderbilt
Debt Instrument [Line Items]
Total floating rate debt	271,229,000			0
Joint venture | 605 West 42nd Street
Debt Instrument [Line Items]
Total floating rate debt	$ 550,000,000			$ 539,000,000
Joint venture | 605 West 42nd Street | Mortgage loan
Debt Instrument [Line Items]
Committed amount			$ 550,000,000
Weighted Average | One Vanderbilt
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)
Weighted Average | Joint venture | 521 Fifth Avenue
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)	3.73%
Weighted Average | Joint venture | 717 Fifth Avenue | Initial Maturity July 2022
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)	4.45%
Weighted Average | Joint venture | 717 Fifth Avenue | Initial Maturity July 2022, 2
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)	5.50%
Weighted Average | Joint venture | 650 Fifth Avenue | Initial Maturity October 2022
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)	4.95%
Weighted Average | Joint venture | 21 East 66th Street
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)	3.60%
Interest rate, floating rate debt (as a percent)	3.62%
Weighted Average | Joint venture | 3 Columbus Circle
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)	3.61%
Weighted Average | Joint venture | 11 Madison Avenue
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)	3.84%
Weighted Average | Joint venture | 800 Third Avenue
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)	3.37%
Weighted Average | Joint venture | East 400 Street 57
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)	3.00%
Weighted Average | Joint venture | Stonehenge Portfolio
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)	4.17%
Interest rate, floating rate debt (as a percent)	2.47%
Weighted Average | Joint venture | 1745 Broadway
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)	3.07%
Weighted Average | Joint venture | 55 West 46th Street
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)	3.52%
Weighted Average | Joint venture | 175-225 Third Street Brooklyn, New York
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)	5.25%
Weighted Average | Joint venture | Jericho Plaza
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)	5.37%
Weighted Average | Joint venture | 724 Fifth Avenue
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)	3.64%
Weighted Average | Joint venture | 1552 Broadway
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)	5.41%
Weighted Average | Joint venture | 280 Park Avenue (9)
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)	3.09%
Weighted Average | Joint venture | 121 Greene Street
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)	2.72%
Weighted Average | Joint venture | 10 East 53rd Street
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)	3.47%
Weighted Average | Joint venture | 131-137 Spring Street
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)	2.77%
Weighted Average | Joint venture | 11 West 34th Street
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)	2.68%
Weighted Average | Joint venture | 100 Park Avenue
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)	2.97%
Weighted Average | Joint venture | One Vanderbilt
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)	4.72%
Weighted Average | Joint venture | 605 West 42nd Street
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)	2.84%
LIBOR | Joint venture | 280 Park Avenue (9) | Mortgage loan
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)			1.48%
LIBOR | Joint venture | 280 Park Avenue (9) | Mezzanine loans
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)			3.85%
LIBOR | Joint venture | 605 West 42nd Street | Mortgage loan
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)			1.44%
Subsequent Event | Joint venture | 55 West 46th Street | Mortgage loan
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)		2.13%
Committed amount		$ 195,000,000
Subsequent Event | Joint venture | 1552 Broadway | Mortgage loan
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)		2.65%
Committed amount		$ 195,000,000