Mortgages and Other Loans Payable (Details)	3 Months Ended				12 Months Ended
	Dec. 31, 2018 USD ($) debt_instrument	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($) debt_instrument extension	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Apr. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jul. 31, 2016 USD ($)
Debt Instrument [Line Items]
Total fixed rate debt	$ 1,389,717,000				$ 1,389,717,000	$ 2,400,560,000
Total floating rate debt	598,442,000				598,442,000	465,431,000
Total fixed rate and floating rate debt	1,988,159,000				1,988,159,000	2,865,991,000
Mortgages reclassed to liabilities related to assets held for sale	0				0	0
Total mortgages and other loans payable	1,988,159,000				1,988,159,000	2,865,991,000
Deferred financing costs, net of amortization	(26,919,000)				(26,919,000)	(28,709,000)
Total mortgages and other loans payable	1,961,240,000				1,961,240,000	2,837,282,000
Loss on early extinguishment of debt	14,889,000	$ 2,194,000	$ 0	$ 0	17,083,000	0	$ 0
Collateral already posted, aggregate fair value	3,900,000,000				3,900,000,000	4,800,000,000
Revolving credit facility, net	492,196,000				492,196,000	30,336,000
Series J Preferred Units
Debt Instrument [Line Items]
Total fixed rate debt	0				0	4,000,000
Unsecured Debt
Debt Instrument [Line Items]
Total fixed rate debt	$ 0				$ 0	16,000,000
Uncommitted Master Repurchase Agreements
Debt Instrument [Line Items]
Debt instrument, number of instruments | debt_instrument	2				2
FHLB Facility
Debt Instrument [Line Items]
Total floating rate debt	$ 13,000,000				$ 13,000,000	0
FHLB Facility | Weighted Average
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)	0.27%				0.27%
2017 Master Repurchase Agreement
Debt Instrument [Line Items]
Total floating rate debt	$ 300,000,000				$ 300,000,000	90,809,000
Initial Term (in Years)					1 year
Number of extension options | extension					2
Period of extension option					1 year
Maximum facility capacity								$ 400,000,000.0	$ 300,000,000.0
Extension option exercised, term					1 year
Revolving credit facility, net	$ 299,600,000				$ 299,600,000
2017 Master Repurchase Agreement | Weighted Average
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)	2.34%				2.34%
FHLB Facility, December 2019
Debt Instrument [Line Items]
Total floating rate debt	$ 14,500,000				$ 14,500,000	0
FHLB Facility, December 2019 | Weighted Average
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)	0.18%				0.18%
2016 Master Repurchase Agreement
Debt Instrument [Line Items]
Initial Term (in Years)					2 years
Period of extension option					1 year
Maximum facility capacity										$ 300,000,000.0
Basis point fee (as a percent)					0.25%
Threshold amount for basis point fee to be applicable (less than)					$ 150,000,000.0
2016 Master Repurchase Agreement | Minimum | LIBOR
Debt Instrument [Line Items]
Credit facility, interest rate (as a percent)					2.25%
2016 Master Repurchase Agreement | Maximum | LIBOR
Debt Instrument [Line Items]
Credit facility, interest rate (as a percent)					4.00%
762 Madison Avenue
Debt Instrument [Line Items]
Total fixed rate debt	$ 771,000				$ 771,000	771,000
762 Madison Avenue | Weighted Average
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)	5.00%				5.00%
100 Church Street
Debt Instrument [Line Items]
Total fixed rate debt	$ 213,208,000				$ 213,208,000	217,273,000
100 Church Street | Weighted Average
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)	4.68%				4.68%
420 Lexington Avenue
Debt Instrument [Line Items]
Total fixed rate debt	$ 300,000,000				$ 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%				3.99%
400 East 58th Street
Debt Instrument [Line Items]
Total fixed rate debt	$ 39,931,000				$ 39,931,000	40,000,000
400 East 58th Street | Weighted Average
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)	3.00%				3.00%
Initial Term (in Years)					5 years
1080 Amsterdam
Debt Instrument [Line Items]
Secured debt, bearing fixed interest, mortgage amount	$ 35,500,000				$ 35,500,000
Secured debt, bearing fixed interest, subordinate loan amount	$ 900,000				$ 900,000
1080 Amsterdam | Weighted Average
Debt Instrument [Line Items]
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%				3.50%
1080 Amsterdam | Senior Subordinated Loans | Weighted Average
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)	7.00%				7.00%
Landmark Square
Debt Instrument [Line Items]
Total fixed rate debt	$ 100,000,000				$ 100,000,000	100,000,000
Landmark Square | Weighted Average
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)	4.90%				4.90%
485 Lexington Avenue
Debt Instrument [Line Items]
Total fixed rate debt	$ 450,000,000				$ 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%				4.25%
1080 Amsterdam Avenue
Debt Instrument [Line Items]
Total fixed rate debt	$ 35,807,000				$ 35,807,000	36,363,000
1080 Amsterdam Avenue | Weighted Average
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)	3.58%				3.58%
315 West 33rd Street
Debt Instrument [Line Items]
Total fixed rate debt	$ 250,000,000				$ 250,000,000	250,000,000
315 West 33rd Street | Weighted Average
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)	4.17%				4.17%
919 Third Avenue
Debt Instrument [Line Items]
Total fixed rate debt	$ 0				$ 0	500,000,000
One Madison Avenue
Debt Instrument [Line Items]
Total fixed rate debt		$ 0				486,153,000
Loss on early extinguishment of debt					14,900,000
113 Greene Street
Debt Instrument [Line Items]
Total floating rate debt	$ 15,523,000				$ 15,523,000	0
113 Greene Street | Weighted Average
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)	2.00%				2.00%
185 Broadway
Debt Instrument [Line Items]
Total floating rate debt	$ 111,869,000				$ 111,869,000	58,000,000
185 Broadway | Weighted Average
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)	2.85%				2.85%
712 Madison Avenue
Debt Instrument [Line Items]
Total floating rate debt	$ 28,000,000				$ 28,000,000	0
712 Madison Avenue | Weighted Average
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)	2.50%				2.50%
115 Spring Street
Debt Instrument [Line Items]
Total floating rate debt	$ 65,550,000				$ 65,550,000	0
115 Spring Street | Weighted Average
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)	3.40%				3.40%
719 Seventh Avenue
Debt Instrument [Line Items]
Total floating rate debt	$ 50,000,000				$ 50,000,000	41,622,000
719 Seventh Avenue | Weighted Average
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)	1.20%				1.20%
220 East 42nd Street
Debt Instrument [Line Items]
Total floating rate debt	$ 0				$ 0	275,000,000
220 East 42nd Street | Weighted Average
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)
Construction Loans | 185 Broadway
Debt Instrument [Line Items]
Initial Term (in Years)					3 years
Number of extension options | extension					2
Period of extension option					1 year
Maximum facility capacity	$ 225,000,000.0				$ 225,000,000.0
Ticonderoga
Debt Instrument [Line Items]
FHLB advances, amount of advances	$ 13,000,000				$ 13,000,000	$ 14,500,000
Ticonderoga | LIBOR
Debt Instrument [Line Items]
Credit facility, interest rate (as a percent)					0.00%	0.00%