Mortgages and Other Loans Payable (Details)	1 Months Ended		3 Months Ended
	Oct. 31, 2021	Jun. 30, 2021	Mar. 31, 2022 USD ($) extension	Dec. 31, 2021 USD ($)	Apr. 30, 2018 USD ($)	Mar. 31, 2017 USD ($)
Debt Instrument [Line Items]
Total fixed rate debt			$ 1,070,571,000	$ 1,073,409,000
Total floating rate debt			313,478,000	361,051,000
Total fixed rate and floating rate debt			1,384,049,000	1,434,460,000
Mortgages reclassed to liabilities related to assets held for sale			(34,348,000)	(34,537,000)
Total mortgages and other loans payable			1,349,701,000	1,399,923,000
Deferred financing costs, net of amortization			(4,926,000)	(5,537,000)
Total mortgages and other loans payable, net			1,344,775,000	1,394,386,000
Outstanding under line of credit facility			491,998,000	381,334,000
Book value of collateral			1,900,000,000	2,100,000,000
2017 Master Repurchase Agreement
Debt Instrument [Line Items]
Total floating rate debt			0	0
Maximum facility capacity			400,000,000		$ 400,000,000	$ 300,000,000
Extension option exercised, term		1 year
Outstanding under line of credit facility			0
100 Church Street
Debt Instrument [Line Items]
Total fixed rate debt			$ 198,980,000	200,212,000
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			$ 287,243,000	288,660,000
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			$ 34,348,000	34,537,000
Total fixed rate debt			33,400,000
Subordinate loan			$ 900,000
1080 Amsterdam Avenue | Weighted Average
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)			3.59%
1080 Amsterdam Avenue | Mortgage loan | Weighted Average
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)			3.50%
Interest rate period			5 years
1080 Amsterdam Avenue | Subordinate loan | Weighted Average
Debt Instrument [Line Items]
Interest rate, fixed rate debt (as a percent)			7.00%
7 Dey / 185 Broadway
Debt Instrument [Line Items]
Total floating rate debt			$ 203,478,000	198,169,000
7 Dey / 185 Broadway | Weighted Average
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)			2.85%
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%
690 Madison Avenue
Debt Instrument [Line Items]
Total floating rate debt			$ 60,000,000	60,000,000
690 Madison Avenue | Weighted Average
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)			1.50%
609 Fifth Avenue
Debt Instrument [Line Items]
Total floating rate debt			$ 0	$ 52,882,000
609 Fifth Avenue | Weighted Average
Debt Instrument [Line Items]
Interest rate, floating rate debt (as a percent)			0.00%
Construction Loans | 185 Broadway
Debt Instrument [Line Items]
Maximum facility capacity			$ 225,000,000
Term (in Years)			3 years
Number of extension options | extension			2
Period of extension options	1 year		1 year