Investments in Unconsolidated Joint Ventures (Details 2) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		6 Months Ended
	Mar. 31, 2015	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Debt Instrument [Line Items]
Total fixed rate debt		$ 4,601,513		$ 4,601,513		$ 4,736,709
Total floating rate debt		856,421		856,421		1,103,942
Total fixed rate and floating rate debt		5,457,934		5,457,934		5,840,651
Loss on early extinguishment of debt	$ (800)	0	$ (1,028)	(49)	$ (1,025)
1745 Broadway
Debt Instrument [Line Items]
Total fixed rate debt		16,000		16,000		16,000
Joint venture
Debt Instrument [Line Items]
Total fixed rate debt		2,851,964		2,851,964		2,065,264
Total floating rate debt		1,508,384		1,508,384		1,713,440
Total fixed rate and floating rate debt		4,360,348		4,360,348		3,778,704
Maximum amount of loan recourse to entity		18,400		18,400
Joint venture | 7 Renaissance
Debt Instrument [Line Items]
Total fixed rate debt		2,600		2,600		2,147
Joint venture | 11 West 34th Street
Debt Instrument [Line Items]
Total fixed rate debt		16,749		16,749		16,905
Joint venture | 280 Park Avenue
Debt Instrument [Line Items]
Total fixed rate debt		696,563		696,563		700,171
Joint venture | 1745 Broadway
Debt Instrument [Line Items]
Total fixed rate debt		340,000		340,000		340,000
Joint venture | Jericho Plaza
Debt Instrument [Line Items]
Total fixed rate debt		163,750		163,750		163,750
Joint venture | 800 Third Avenue
Debt Instrument [Line Items]
Total fixed rate debt		20,910		20,910		20,910
Joint venture | 315 West 36th Street
Debt Instrument [Line Items]
Total fixed rate debt		25,000		25,000		25,000
Joint venture | 521 Fifth Avenue
Debt Instrument [Line Items]
Total fixed rate debt		170,000		170,000		170,000
Joint venture | 717 Fifth Avenue | Mortgage loan
Debt Instrument [Line Items]
Committed amount		300,000		300,000
Joint venture | 717 Fifth Avenue | Mezzanine loans
Debt Instrument [Line Items]
Committed amount		290,000		290,000
Joint venture | 21 East 66th Street
Debt Instrument [Line Items]
Total fixed rate debt		12,000		12,000		12,000
Total floating rate debt		1,844		1,844		1,883
Joint venture | 3 Columbus Circle
Debt Instrument [Line Items]
Total fixed rate debt		350,000		350,000		0
Total floating rate debt		0		0		230,974
Joint venture | Stonehenge Portfolio
Debt Instrument [Line Items]
Total fixed rate debt		434,492		434,492		0
Total floating rate debt		10,500		10,500		0
Joint venture | The Meadows
Debt Instrument [Line Items]
Total floating rate debt		67,350		67,350		67,350
Joint venture | 1552 Broadway
Debt Instrument [Line Items]
Total floating rate debt		188,410		188,410		184,210
Joint venture | 1552 Broadway | Mortgage loan
Debt Instrument [Line Items]
Committed amount		150,000		150,000
Unfunded amount		1,700		1,700
Joint venture | 1552 Broadway | Mezzanine loans
Debt Instrument [Line Items]
Committed amount		41,500		41,500
Unfunded amount		1,400		1,400
Joint venture | Other loan payable
Debt Instrument [Line Items]
Total floating rate debt		30,000		30,000		30,000
Joint venture | 650 Fifth Avenue
Debt Instrument [Line Items]
Total floating rate debt		65,000		65,000		65,000
Joint venture | 650 Fifth Avenue | Mortgage loan
Debt Instrument [Line Items]
Committed amount		97,000		97,000
Unfunded amount		32,000		32,000
Joint venture | 175-225 Third Street
Debt Instrument [Line Items]
Total floating rate debt		40,000		40,000		40,000
Joint venture | 10 East 53rd Street
Debt Instrument [Line Items]
Total floating rate debt		125,000		125,000		125,000
Joint venture | 724 Fifth Avenue
Debt Instrument [Line Items]
Total floating rate debt		275,000		275,000		275,000
Joint venture | 33 Beekman
Debt Instrument [Line Items]
Total floating rate debt		65,506		65,506		52,283
Maximum amount of loan recourse to entity		75,000		75,000
Joint venture | 600 Lexington Avenue
Debt Instrument [Line Items]
Total floating rate debt		114,774		114,774		116,740
Joint venture | 55 West 46th Street
Debt Instrument [Line Items]
Total floating rate debt		150,000		150,000		150,000
Joint venture | 55 West 46th Street | Mortgage loan
Debt Instrument [Line Items]
Committed amount		190,000		190,000
Unfunded amount		40,000		40,000
Joint venture | 121 Greene Street
Debt Instrument [Line Items]
Total floating rate debt		15,000		15,000		15,000
Joint venture | 100 Park Avenue
Debt Instrument [Line Items]
Total floating rate debt		360,000		360,000		360,000
Initial Maturity July 2022 | Joint venture | 717 Fifth Avenue
Debt Instrument [Line Items]
Total fixed rate debt		300,000		300,000		300,000
Initial Maturity July 2024 | Joint venture | 717 Fifth Avenue
Debt Instrument [Line Items]
Total fixed rate debt		319,900		319,900		$ 314,381
Initial Maturity July 2016 | Joint venture | Stonehenge Portfolio
Debt Instrument [Line Items]
Total fixed rate debt		13,500		13,500
Initial Maturity June 2017 | Joint venture | Stonehenge Portfolio
Debt Instrument [Line Items]
Total fixed rate debt		56,000		56,000
Initial Maturity November 2017 | Joint venture | Stonehenge Portfolio
Debt Instrument [Line Items]
Total fixed rate debt		35,000		35,000
Initial Maturity February 2018 | Joint venture | Stonehenge Portfolio
Debt Instrument [Line Items]
Total fixed rate debt		7,400		7,400
Initial Maturity August 2019 | Joint venture | Stonehenge Portfolio
Debt Instrument [Line Items]
Total fixed rate debt		142,700		142,700
Initial Maturity June 2024 | Joint venture | Stonehenge Portfolio
Debt Instrument [Line Items]
Total fixed rate debt		$ 179,900		$ 179,900
Weighted Average | 1745 Broadway
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)		4.81%		4.81%
Weighted Average | Joint venture | 7 Renaissance
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)		10.00%		10.00%
Weighted Average | Joint venture | 11 West 34th Street
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)		4.82%		4.82%
Weighted Average | Joint venture | 280 Park Avenue
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)		6.57%		6.57%
Weighted Average | Joint venture | 1745 Broadway
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)		5.68%		5.68%
Weighted Average | Joint venture | Jericho Plaza
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)		5.65%		5.65%
Weighted Average | Joint venture | 800 Third Avenue
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)		6.00%		6.00%
Weighted Average | Joint venture | 315 West 36th Street
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)		3.16%		3.16%
Weighted Average | Joint venture | 521 Fifth Avenue
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)		3.73%		3.73%
Weighted Average | Joint venture | 21 East 66th Street
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)		3.60%		3.60%
Interest rate, floating rate debt (as a percent)		2.88%		2.88%
Weighted Average | Joint venture | 3 Columbus Circle
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)		3.61%		3.61%
Weighted Average | Joint venture | Stonehenge Portfolio
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)		4.18%		4.18%
Interest rate, floating rate debt (as a percent)		3.25%		3.25%
Weighted Average | Joint venture | The Meadows
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)		7.75%		7.75%
Weighted Average | Joint venture | 1552 Broadway
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)		4.26%		4.26%
Weighted Average | Joint venture | Other loan payable
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)		1.08%		1.08%
Weighted Average | Joint venture | 650 Fifth Avenue
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)		3.69%		3.69%
Weighted Average | Joint venture | 175-225 Third Street
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)		4.25%		4.25%
Weighted Average | Joint venture | 10 East 53rd Street
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)		2.69%		2.69%
Weighted Average | Joint venture | 724 Fifth Avenue
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)		2.60%		2.60%
Weighted Average | Joint venture | 33 Beekman
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)		2.93%		2.93%
Weighted Average | Joint venture | 600 Lexington Avenue
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)		2.28%		2.28%
Weighted Average | Joint venture | 55 West 46th Street
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)		2.49%		2.49%
Weighted Average | Joint venture | 121 Greene Street
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)		1.69%		1.69%
Weighted Average | Joint venture | 100 Park Avenue
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)		1.94%		1.94%
Weighted Average | Initial Maturity July 2022 | Joint venture | 717 Fifth Avenue
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)		4.45%		4.45%
Weighted Average | Initial Maturity July 2024 | Joint venture | 717 Fifth Avenue
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)		9.00%		9.00%