Mortgages and Other Loans Payable (Details)	1 Months Ended	3 Months Ended
	Jun. 30, 2020	Mar. 31, 2021 USD ($) extension	Dec. 31, 2020 USD ($)	Apr. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)
Debt Instrument [Line Items]
Total fixed rate debt		$ 1,081,142,000	$ 1,083,683,000
Total floating rate debt		786,521,000	917,677,000
Total mortgages and other loans payable		1,867,663,000	2,001,360,000
Deferred financing costs, net of amortization		(18,962,000)	(21,388,000)
Total mortgages and other loans payable, net		1,848,701,000	1,979,972,000
Book value of collateral		2,400,000,000	2,500,000,000
Outstanding under line of credit facility		625,809,000	105,262,000
FHLB Facility, January 2021, 1
Debt Instrument [Line Items]
Total floating rate debt		0	10,000,000
FHLB Facility, January 2021, 2
Debt Instrument [Line Items]
Total floating rate debt		0	15,000,000
FHLB Facility January 2021
Debt Instrument [Line Items]
Total floating rate debt		0	35,000,000
2017 Master Repurchase Agreement
Debt Instrument [Line Items]
Total floating rate debt		0	0
Maximum facility capacity		400,000,000		$ 400,000,000.0	$ 300,000,000.0
Margin calls		$ 0
Extension option exercised, term	1 year	1 year
Outstanding under line of credit facility		$ 0
100 Church Street
Debt Instrument [Line Items]
Total fixed rate debt		$ 203,697,000	204,875,000
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		$ 292,672,000	294,035,000
420 Lexington Avenue | Weighted Average
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)		3.99%
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		$ 34,773,000	34,773,000
Total fixed rate debt		33,900,000
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%
185 Broadway
Debt Instrument [Line Items]
Total floating rate debt		$ 168,870,000	158,478,000
185 Broadway | Weighted Average
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)		2.85%
609 Fifth Avenue
Debt Instrument [Line Items]
Total floating rate debt		$ 57,651,000	57,651,000
609 Fifth Avenue | Weighted Average
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)		2.95%
220 East 42nd Street
Debt Instrument [Line Items]
Total floating rate debt		$ 510,000,000	510,000,000
220 East 42nd Street | Weighted Average
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)		2.75%
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%
133 Greene Street
Debt Instrument [Line Items]
Total floating rate debt		$ 0	15,523,000
106 Spring Street
Debt Instrument [Line Items]
Total floating rate debt		0	38,025,000
712 Madison Avenue
Debt Instrument [Line Items]
Total floating rate debt		$ 0	$ 28,000,000
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