Mortgages and Other Loans Payable (Details)	1 Months Ended			12 Months Ended
	Feb. 22, 2018	Jan. 31, 2018	Nov. 30, 2017	Dec. 31, 2017 USD ($) extenstion_option debt_instrument $ / shares	Jun. 30, 2017 USD ($)	Jan. 31, 2017 partner	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jul. 31, 2016 USD ($)
Debt Instrument [Line Items]
Total fixed rate debt				$ 2,400,560,000			$ 3,518,127,000
Total floating rate debt				465,431,000			622,585,000
Total fixed rate and floating rate debt				2,865,991,000			4,140,712,000
Mortgages reclassed to liabilities related to assets held for sale				0			0
Total mortgages and other loans payable				2,865,991,000			4,140,712,000
Total mortgages and other loans payable				$ 2,837,282,000			4,073,830,000
Number of extension options | extenstion_option				2
Debt Instrument, period of extension option				1 year
Revolving credit facility, net				$ 30,336,000			0
Deferred financing costs, net				28,709,000			66,882,000
Collateral already posted, aggregate fair value				4,800,000,000			6,000,000,000
Series J Preferred Units
Debt Instrument [Line Items]
Total fixed rate debt				4,000,000			4,000,000
Temporary equity, carrying amount, attributable to noncontrolling interest				$ 4,000,000
Preferred Units (as a percent)				3.75%
Preferred Units, liquidation preference (in dollars per unit) | $ / shares				$ 1,000.00
Series J Preferred Units | Weighted Average
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)				3.75%
FHLB Advance Maturing in January 2017
Debt Instrument [Line Items]
Total fixed rate debt				$ 0			105,000,000
FHLB Advance Maturing in January 2017, 2
Debt Instrument [Line Items]
Total fixed rate debt				0			100,000,000
Uncommitted Master Repurchase Agreement 2017
Debt Instrument [Line Items]
Total floating rate debt				$ 90,809,000			0
Initial Term (in Years)				1 year
Maximum facility capacity					$ 300,000,000.0
Revolving credit facility, net				$ 90,100,000
Uncommitted Master Repurchase Agreement 2017 | Weighted Average
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)				2.38%
Unsecured Debt
Debt Instrument [Line Items]
Total fixed rate debt				$ 16,000,000			16,000,000
Unsecured Debt | Weighted Average
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)				4.81%
Master Repurchase Agreement
Debt Instrument [Line Items]
Total floating rate debt				$ 0			184,642,000
Initial Term (in Years)				2 years
Maximum facility capacity									$ 300,000,000.0
Debt Instrument, period of extension option				1 year
Revolving credit facility, net				$ (1,200,000)
Basis Point Fee (as a percent)				0.25%
Threshold amount for basis point fee to be applicable (less than)				$ 150,000,000.0
Master Repurchase Agreement | Minimum | LIBOR
Debt Instrument [Line Items]
Credit facility, interest rate (as a percent)				2.25%
Master Repurchase Agreement | Maximum | LIBOR
Debt Instrument [Line Items]
Credit facility, interest rate (as a percent)				4.00%
Uncommitted Master Repurchase Agreements
Debt Instrument [Line Items]
Debt instrument, number of instruments | debt_instrument				2
485 Lexington Avenue
Debt Instrument [Line Items]
Total fixed rate debt				$ 450,000,000			450,000,000
485 Lexington Avenue | Weighted Average
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)				4.25%
762 Madison Avenue
Debt Instrument [Line Items]
Total fixed rate debt				$ 771,000			7,694,000
762 Madison Avenue | Weighted Average
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)				5.00%
One Madison Avenue
Debt Instrument [Line Items]
Total fixed rate debt				$ 486,153,000			517,806,000
One Madison Avenue | Weighted Average
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)				5.91%
100 Church Street
Debt Instrument [Line Items]
Total fixed rate debt				$ 217,273,000			221,446,000
100 Church Street | Weighted Average
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)				4.68%
919 Third Avenue
Debt Instrument [Line Items]
Total fixed rate debt				$ 500,000,000			500,000,000
Interest in property (as a percent)				51.00%
919 Third Avenue | Weighted Average
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)				5.12%
420 Lexington Avenue
Debt Instrument [Line Items]
Total fixed rate debt				$ 300,000,000			300,000,000
420 Lexington Avenue | Weighted Average
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)				3.99%
1515 Broadway
Debt Instrument [Line Items]
Total fixed rate debt				$ 0			888,531,000
400 East 58th Street
Debt Instrument [Line Items]
Total fixed rate debt				$ 40,000,000			40,000,000
400 East 58th Street | Weighted Average
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)				3.00%
Initial Term (in Years)				5 years
Landmark Square
Debt Instrument [Line Items]
Total fixed rate debt				$ 100,000,000			100,000,000
Landmark Square | Weighted Average
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)				4.90%
885 Third Avenue
Debt Instrument [Line Items]
Total fixed rate debt				$ 0			267,650,000
885 Third Avenue | Weighted Average
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)
719 Seventh Avenue
Debt Instrument [Line Items]
Total floating rate debt				$ 41,622,000			37,388,000
719 Seventh Avenue | Weighted Average
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)				3.05%
183, 187 Broadway & 5-7 Dey Street
Debt Instrument [Line Items]
Total floating rate debt				$ 58,000,000			58,000,000
183, 187 Broadway & 5-7 Dey Street | Weighted Average
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)				2.70%
1080 Amsterdam
Debt Instrument [Line Items]
Total fixed rate debt				$ 36,363,000			0
Total floating rate debt				0			3,525,000
Secured debt, bearing fixed interest, mortgage amount				35,500,000
Secured debt, bearing fixed interest, subordinate loan amount				$ 900,000
1080 Amsterdam | Weighted Average
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)				3.58%
Initial Term (in Years)				5 years
1080 Amsterdam | Mortgage loan | Weighted Average
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)				3.50%
1080 Amsterdam | Senior Subordinated Loans | Weighted Average
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)				7.00%
315 West 33rd Street
Debt Instrument [Line Items]
Total fixed rate debt				$ 250,000,000			0
315 West 33rd Street | Weighted Average
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)				4.17%
220 East 42nd Street
Debt Instrument [Line Items]
Total floating rate debt				$ 275,000,000			275,000,000
220 East 42nd Street | Weighted Average
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)				1.60%
One Vanderbilt
Debt Instrument [Line Items]
Total floating rate debt				$ 0			$ 64,030,000
Number of joint venture partners | partner						2
One Vanderbilt | Construction Loans
Debt Instrument [Line Items]
Maximum facility capacity								$ 1,500,000,000.0
Number of joint venture partners | partner						2
Subsequent Event | 719 Seventh Avenue
Debt Instrument [Line Items]
Debt Instrument, period of extension option		1 year
Allianz Real Estate | 1515 Broadway
Debt Instrument [Line Items]
Ownership percentage in disposed asset			30.13%
Allianz Real Estate | Subsequent Event | 1515 Broadway
Debt Instrument [Line Items]
Ownership percentage in disposed asset	12.87%