Investments in Partially Owned Entities (Farley Post Office, NY Mezzanine Loan, Suffolk Downs) (Details) ft² in Thousands		1 Months Ended		12 Months Ended
	May 09, 2017 USD ($)	Jun. 15, 2017 USD ($) myExtension	May 26, 2017 USD ($)	Dec. 31, 2017 USD ($) ft²	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2017	Sep. 30, 2016
Schedule of Equity Method Investments [Line Items]
Proceeds from the repayment of JBG SMITH Properties loan receivable				$ 9,543,000	$ 183,173,000	$ 786,924,000
Net gains on sale of real estate and a lease position				$ 3,489,000	$ 5,074,000	$ 65,396,000
Farley Post Office Building
Schedule of Equity Method Investments [Line Items]
Equity method ownership percentage								50.10%
Upfront contribution		$ 115,230,000
Farley Post Office Building | Farley Post Office Joint Venture
Schedule of Equity Method Investments [Line Items]
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		$ 210,269,000
Mezzanine Loan New York
Schedule of Equity Method Investments [Line Items]
Equity method ownership percentage	33.30%
Gain on debt investments	$ 50,000,000
Suffolk Downs
Schedule of Equity Method Investments [Line Items]
Gain on debt investments				11,373,000
Net gains on sale of real estate and a lease position				$ 15,314,000
Suffolk Downs | Suffolk Downs JV
Schedule of Equity Method Investments [Line Items]
Equity method ownership percentage			21.20%
Gain on debt investments			$ 11,373,000
Proceeds from the repayment of JBG SMITH Properties loan receivable			155,000,000
Net gains on sale of real estate and a lease position			15,314,000
Principal received			29,318,000
Accrued interest on our debt investment			$ 6,129,000
Office | Farley Post Office Building | Farley Post Office Joint Venture
Schedule of Equity Method Investments [Line Items]
Net rentable area | ft²				730
Retail | Farley Post Office Building | Farley Post Office Joint Venture
Schedule of Equity Method Investments [Line Items]
Net rentable area | ft²				120
Interest Only Loan Maturing in June 2019 | Farley Post Office Building | Farley Post Office Joint Venture
Schedule of Equity Method Investments [Line Items]
Debt instrument, interest rate, effective percentage							4.64%
Number of loan extensions available | myExtension		2
Length of extension available		1 year
LIBOR | Interest Only Loan Maturing in June 2019 | Farley Post Office Building | Farley Post Office Joint Venture
Schedule of Equity Method Investments [Line Items]
Spread Over LIBOR (in percentage)		3.25%
Joint Venture | Mezzanine Loan New York
Schedule of Equity Method Investments [Line Items]
Loans payable	$ 150,000,000
Joint Venture | LIBOR | Mezzanine Loan | Mezzanine Loan New York
Schedule of Equity Method Investments [Line Items]
Spread Over LIBOR (in percentage)	9.42%