Investments in Partially Owned Entities (Independence Plaza, Toys R Us and 666 5th Avenue) (Details)				1 Months Ended	3 Months Ended			6 Months Ended
	Jun. 11, 2018 USD ($)	May 09, 2018 USD ($) ft	Apr. 05, 2018 USD ($)	Dec. 31, 2012 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) building unit	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Schedule Of Equity Method Investments
Noncontrolling interests in controlling subsidiaries					$ (26,175,000)		$ 7,677,000	$ (34,449,000)	$ 14,414,000
Net gains on disposition of wholly owned and partially owned assets					23,559,000		$ 0	23,559,000	$ 501,000
Mortgages payable, net					$ 8,108,618,000			$ 8,108,618,000		$ 8,137,139,000
Independence Plaza
Schedule Of Equity Method Investments
Equity method ownership percentage					50.10%			50.10%
Number of buildings | building								3
Number of units | unit								1,327
Real estate transfer tax				$ 1,730,000
Increase in accrued taxes payable						$ 13,103,000
Refinancing	$ 675,000,000
Interest rate, effective	3.48%
Proceeds from (repayments of) debt	$ 550,000,000
Net proceeds from debt	$ 55,618,000
Toys R Us
Schedule Of Equity Method Investments
Equity method ownership percentage					32.50%			32.50%
Equity method investment, carrying amount					$ 0			$ 0
Tax basis of investments, cost for income tax purposes					420,000,000			420,000,000
Transaction Related Costs and Other | Independence Plaza
Schedule Of Equity Method Investments
Increase in accrued taxes payable			$ 13,103,000
Noncontrolling interests in controlling subsidiaries								6,538,000
Loans Maturing in July 2025 | Independence Plaza
Schedule Of Equity Method Investments
Debt term	7 years
Debt instrument, interest rate, stated percentage	4.25%
Office | 666 Fifth Avenue
Schedule Of Equity Method Investments
Equity method ownership percentage		49.50%
Net proceeds from debt					58,000,000
Proceeds from the sale of real estate		$ 120,000,000
Net gains on disposition of wholly owned and partially owned assets		134,000,000
Gain on sale of properties, net		$ 244,000,000
Linear feet of frontage | ft		125
Mortgages payable, net					$ 1,400,000,000			$ 1,400,000,000