Subsequent Events (Details) - USD ($)	Oct. 09, 2015	Oct. 08, 2015	Oct. 07, 2015	Sep. 30, 2015	Dec. 31, 2014
Subsequent Event [Line Items]
Unsecured term loans				$ 200,000,000	$ 200,000,000
Mortgages [Member] | Tysons Corner [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Repayments of debt	$ 88,400,000
Accrued interest	$ 400,000
Unsecured Debt [Member] | Seven Year Term Loan [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Debt term	7 years
Unsecured term loans		$ 250,000,000	$ 200,000,000
Increase in debt		50,000,000
Increase in limit		$ 150,000,000
Effective interest rate		3.72%
LIBOR [Member] | Unsecured Debt [Member] | Seven Year Term Loan [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Basis spread on variable rate		1.80%