Investments in Partially Owned Entities (Farley Post Office, NY Mezzanine Loan, Suffolk Downs) (Details) ft² in Thousands			1 Months Ended		3 Months Ended	9 Months Ended
	Jul. 17, 2017	May 09, 2017 USD ($)	Jun. 15, 2017 USD ($) myExtension	May 26, 2017 USD ($)	Jun. 30, 2017 USD ($)	Sep. 30, 2017 USD ($) ft²	Sep. 30, 2016 USD ($)
Schedule Of Equity Method Investments
Gain on debt investments						$ (11,373,000)
Proceeds from sales of real estate and related investments						9,543,000	$ 167,673,000
Gain (loss) on sale of properties						$ 3,797,000	$ 5,074,000
LIBOR
Schedule Of Equity Method Investments
Spread Over LIBOR (in percentage)	1.25%
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
Length of lease			99 years
Upfront contribution			$ 230,000,000
Annual rent payments			5,000,000
Construction Loan			271,000,000
Initial advance to construction			$ 202,299,000
Description of variable rate basis			LIBOR
Debt instrument maturity			2019-06
Farley post office building | Farley Post Office JV | Office
Schedule Of Equity Method Investments
Net rentable area | ft²						730
Farley post office building | Farley Post Office JV | Retail
Schedule Of Equity Method Investments
Net rentable area | ft²						120
Mezzanine Loan - New York
Schedule Of Equity Method Investments
Equity method ownership percentage		33.30%
Gain on debt investments		$ 50,000,000
Suffolk Downs
Schedule Of Equity Method Investments
Gain on debt investments					$ 11,373,000
Gain (loss) on sale of properties					$ 15,314,000
Suffolk Downs | Suffolk Downs JV
Schedule Of Equity Method Investments
Equity method ownership percentage				21.20%
Gain on debt investments				$ 11,373,000
Proceeds from sales of real estate and related investments				155,000,000
Gain (loss) on sale of properties				15,314,000
Principal received				29,318,000
Accrued interest on our debt investment				$ 6,129,000
Joint Venture | Mezzanine Loan - New York
Schedule Of Equity Method Investments
Description of variable rate basis		LIBOR
Loans payable		$ 150,000,000
Debt instrument maturity		2017-05
Interest Only Loan Maturing in June 2019 | Farley post office building | Farley Post Office JV
Schedule Of Equity Method Investments
Number of loan extensions available | myExtension			2
Length of extension available			1 year
Interest Only Loan Maturing in June 2019 | Farley post office building | Farley Post Office JV | LIBOR
Schedule Of Equity Method Investments
Spread Over LIBOR (in percentage)			3.25%
Interest rate, effective						4.48%
Mezzanine Loan | Joint Venture | Mezzanine Loan - New York | LIBOR
Schedule Of Equity Method Investments
Spread Over LIBOR (in percentage)		9.42%