Investments in Partially Owned Entities (Summary of Debt) (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	May 31, 2016	Dec. 31, 2017	Dec. 31, 2016
Partially Owned Office Buildings
Schedule of Equity Method Investments [Line Items]
Debt instrument, interest rate, effective percentage		3.76%
Maturity, range, start		2019
Maturity, range, end			2026
Equity method investment debt of partially owned entities		$ 3,934,894	$ 3,227,053
PREIT
Schedule of Equity Method Investments [Line Items]
Equity method ownership percentage		8.00%
Debt instrument, interest rate, effective percentage		3.61%
Maturity, range, start		2018
Maturity, range, end			2025
Equity method investment debt of partially owned entities		$ 1,586,045	$ 1,747,543
Urban Edge Properties
Schedule of Equity Method Investments [Line Items]
Equity method ownership percentage		4.50%	5.40%
Debt instrument, interest rate, effective percentage		4.11%
Maturity, range, start		2018
Maturity, range, end			2034
Equity method investment debt of partially owned entities		$ 1,415,806	$ 1,209,994
Alexanders Inc
Schedule of Equity Method Investments [Line Items]
Equity method ownership percentage		32.40%
Debt instrument, interest rate, effective percentage		2.61%
Maturity, range, start		2018
Maturity, range, end			2022
Equity method investment debt of partially owned entities		$ 1,252,440	$ 1,056,147
Other equity method investments
Schedule of Equity Method Investments [Line Items]
Debt instrument, interest rate, effective percentage		7.73%
Maturity, range, start		2018
Maturity, range, end			2023
Equity method investment debt of partially owned entities		$ 8,601,383	$ 8,540,710
7 West 34th Street
Schedule of Equity Method Investments [Line Items]
Sale of ownership interest percent	47.00%
Mortgage loans on real estate, face amount of mortgages		300,000
Partially Owned Properties
Schedule of Equity Method Investments [Line Items]
Equity method investment pro rata share debt of partially owned entities		$ 5,288,276	$ 4,895,497