Investments in Partially Owned Entities (Farley Post Office, NY Mezzanine Loan, Suffolk Downs) (Details) ft² in Thousands		1 Months Ended		3 Months Ended	6 Months Ended
	May 09, 2017 USD ($)	Jun. 15, 2017 USD ($) Extensions	May 26, 2017 USD ($)	Jun. 30, 2017 USD ($) ft²	Jun. 30, 2017 USD ($) ft²	Jun. 30, 2016 USD ($)	Sep. 30, 2016
Schedule Of Equity Method Investments
Proceeds from sales of real estate					$ 5,180,000	$ 159,888,000
Gain (loss) on sale of property					$ 2,267,000	$ 2,210,000
Mezzanine Loan - New York
Schedule Of Equity Method Investments
Equity method ownership percentage	33.30%
Gain on debt investments	$ 50,000,000
Farley post office building
Schedule Of Equity Method Investments
Equity method ownership percentage							50.10%
Upfront contribution		$ 115,230,000
Farley post office building | Farley Post Office JV
Schedule Of Equity Method Investments
Net Rentable Area | ft²				850	850
Length of lease		99 years
Loan Extension term		1 year
Number of loan extensions available | Extensions		2
Debt Instrument Maturity		2019-06
Description of variable rate basis		LIBOR
Interest Rate, Effective				4.41%	4.41%
Upfront contribution		$ 230,000,000
Construction Loan		271,000,000
Initial advance to construction		202,299,000
Annual rent payments		$ 5,000,000
Farley post office building | Farley Post Office JV | Office
Schedule Of Equity Method Investments
Net Rentable Area | ft²				730	730
Farley post office building | Farley Post Office JV | Retail
Schedule Of Equity Method Investments
Net Rentable Area | ft²				120	120
Farley post office building | Farley Post Office JV | LIBOR
Schedule Of Equity Method Investments
Spread Over LIBOR (in percentage)		3.25%
Suffolk Downs
Schedule Of Equity Method Investments
Equity method ownership percentage			21.20%
Gain on debt investments			$ 11,373,000	$ 11,373,000
Gain (loss) on sale of property			15,314,000	$ 15,314,000
Accrued interest on our debt investment			6,129,000
Principal received			29,318,000
Suffolk Downs | Suffolk Downs JV
Schedule Of Equity Method Investments
Proceeds from sales of real estate			$ 155,000,000
Joint Venture | Mezzanine Loan - New York
Schedule Of Equity Method Investments
Loans Payable	$ 150,000,000
Debt Instrument Maturity	2017-05
Description of variable rate basis	LIBOR
Joint Venture | Mezzanine Loan - New York | LIBOR
Schedule Of Equity Method Investments
Spread Over LIBOR (in percentage)	9.42%