Mortgages and Other Loans Payable (Details)	1 Months Ended			9 Months Ended
	Aug. 31, 2019 extension	Jun. 30, 2019	Jun. 30, 2018	Sep. 30, 2019 USD ($) extension	Dec. 31, 2018 USD ($)	Apr. 30, 2018 USD ($)
Debt Instrument [Line Items]
Total fixed rate debt				$ 1,385,756,000	$ 1,389,717,000
Total floating rate debt				1,068,927,000	598,442,000
Total mortgages and other loans payable				2,454,683,000	1,988,159,000
Deferred financing costs, net of amortization				(37,522,000)	(26,919,000)
Total mortgages and other loans payable, net				2,417,161,000	1,961,240,000
Book value of collateral				3,700,000,000	2,600,000,000
Outstanding under line of credit facility				328,549,000	492,196,000
2017 Master Repurchase Agreement
Debt Instrument [Line Items]
Total floating rate debt				190,107,000	300,000,000
Maximum facility capacity				300,000,000.0		$ 400,000,000.0
Number of extension options | extension	2
Extension option exercised, term	1 year	1 year	1 year
Outstanding under line of credit facility				$ 189,600,000
2017 Master Repurchase Agreement | Weighted Average
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)				2.22%
FHLB Facility, December 2019
Debt Instrument [Line Items]
Total floating rate debt				$ 14,500,000	14,500,000
FHLB Facility, December 2019 | Weighted Average
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)				0.18%
FHLB Facility, January 2020
Debt Instrument [Line Items]
Total floating rate debt				$ 10,000,000	0
FHLB Facility, January 2020 | Weighted Average
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)				0.26%
FHLB Facility
Debt Instrument [Line Items]
Total floating rate debt				$ 0	13,000,000
FHLB Facility, February 2020
Debt Instrument [Line Items]
Total floating rate debt				$ 15,000,000	0
FHLB Facility, February 2020 | Weighted Average
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)				0.32%
762 Madison Avenue
Debt Instrument [Line Items]
Total fixed rate debt				$ 771,000	771,000
762 Madison Avenue | Weighted Average
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)				5.00%
100 Church Street
Debt Instrument [Line Items]
Total fixed rate debt				$ 210,382,000	213,208,000
100 Church Street | Weighted Average
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)				4.68%
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%
400 East 58th Street
Debt Instrument [Line Items]
Total fixed rate debt				$ 39,306,000	39,931,000
400 East 58th Street | Weighted Average
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)				3.00%
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%
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%
1080 Amsterdam Avenue
Debt Instrument [Line Items]
Total fixed rate debt				$ 35,297,000	35,807,000
Total fixed rate debt				35,500,000
Subordinate loan				$ 900,000
1080 Amsterdam Avenue | Weighted Average
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)				3.59%
Term (in Years)				5 years
1080 Amsterdam Avenue | Mortgage loan | Weighted Average
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)				3.50%
1080 Amsterdam Avenue | Subordinate loan | 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	250,000,000
315 West 33rd Street | Weighted Average
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)				4.17%
113 Greene Street
Debt Instrument [Line Items]
Total floating rate debt				$ 15,523,000	15,523,000
113 Greene Street | Weighted Average
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)				2.00%
106 Spring Street
Debt Instrument [Line Items]
Total floating rate debt				$ 38,025,000	0
106 Spring Street | Weighted Average
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)				2.50%
609 Fifth Avenue
Debt Instrument [Line Items]
Total floating rate debt				$ 52,990,000	0
609 Fifth Avenue | Weighted Average
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)				2.40%
185 Broadway
Debt Instrument [Line Items]
Total floating rate debt				$ 115,886,000	111,869,000
185 Broadway | Weighted Average
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)				2.85%
712 Madison Avenue
Debt Instrument [Line Items]
Total floating rate debt				$ 28,000,000	28,000,000
712 Madison Avenue | Weighted Average
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)				2.50%
460 West 34th Street
Debt Instrument [Line Items]
Total floating rate debt				$ 310,236,000
460 West 34th Street | Weighted Average
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)				2.23%
Suburban Loan
Debt Instrument [Line Items]
Total floating rate debt				$ 228,660,000	0
Suburban Loan | Weighted Average
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)				2.79%
719 Seventh Avenue
Debt Instrument [Line Items]
Total floating rate debt				$ 50,000,000	50,000,000
719 Seventh Avenue | Weighted Average
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)				1.20%
115 Spring Street
Debt Instrument [Line Items]
Total floating rate debt				$ 0	$ 65,550,000
Construction Loans | 185 Broadway
Debt Instrument [Line Items]
Term (in Years)				3 years
Maximum facility capacity				$ 225,000,000.0
Number of extension options | extension				2
Period of extension options				1 year
Construction Loans | 460 West 34th Street
Debt Instrument [Line Items]
Term (in Years)				3 years
Maximum facility capacity				$ 465,000,000.0
Number of extension options | extension				2
Period of extension options				1 year
LIBOR Plus 18 Basis Points | Ticonderoga Insurance Company
Debt Instrument [Line Items]
FHLB advances				$ 14,500,000
LIBOR Plus 18 Basis Points | Ticonderoga Insurance Company | LIBOR
Debt Instrument [Line Items]
Interest rate added to base rate (as a percent)				0.18%
LIBOR Plus 26 Basis Points | Ticonderoga Insurance Company
Debt Instrument [Line Items]
FHLB advances				$ 10,000,000.0
LIBOR Plus 26 Basis Points | Ticonderoga Insurance Company | LIBOR
Debt Instrument [Line Items]
Interest rate added to base rate (as a percent)				0.26%
LIBOR Plus 32 Basis Points | Ticonderoga Insurance Company
Debt Instrument [Line Items]
FHLB advances				$ 15,000,000.0
LIBOR Plus 32 Basis Points | Ticonderoga Insurance Company | LIBOR
Debt Instrument [Line Items]
Interest rate added to base rate (as a percent)				0.32%
115 Spring Street
Debt Instrument [Line Items]
Ownership percentage in disposed asset	49.00%
Deconsolidation of ownership interest, percentage	51.00%