Investments in Partially Owned Entities (Alexander's Inc.) (Details)	Sep. 14, 2020 USD ($)	Dec. 31, 2020 USD ($) $ / shares shares	Oct. 23, 2020 USD ($) apartment_unit	Sep. 13, 2020 USD ($)	Dec. 31, 2018 USD ($)
Alexanders Inc
Equity Method Investments And Income From Equity Method Investments
Investment owned, balance, shares | shares		1,654,068
Equity method ownership percentage		32.40%
Accounts receivable, related parties		$ 1,516,000			$ 1,426,000
Closing share price (in dollars per share) | $ / shares		$ 277.35
Equity method investment fair value		$ 458,756,000
Excess of investee's fair value over carrying amount		375,854,000
Excess of investee's carrying amount over equity in net assets		$ 38,470,000
Interest rate, end of period (percent)		1.65%
Mortgages
Equity Method Investments And Income From Equity Method Investments
Interest rate, end of period (percent)		2.91%
Lexington Avenue Property | Mortgages | Retail | Alexanders Inc
Equity Method Investments And Income From Equity Method Investments
Debt instrument, amount	$ 300,000,000			$ 350,000,000
Repayment of debt	$ 50,000,000
Spread Over LIBOR (in percentage)	1.40%
Interest rate, end of period (percent)		1.55%
Debt instrument, interest rate, stated percentage		1.72%
The Alexander | Mortgages | Residential | Alexanders Inc
Equity Method Investments And Income From Equity Method Investments
Notes payable gross			$ 94,000,000
Number of units | apartment_unit			312
Debt instrument, interest rate, stated percentage			2.63%