Term Loan (Details) - USD ($)	1 Months Ended		6 Months Ended
	Jun. 30, 2015	Jan. 31, 2013	Jun. 30, 2015	Dec. 31, 2014
Credit facility
Term loans	$ 320,000,000		$ 320,000,000	$ 70,000,000
Unsecured debt
Credit facility
Term loans	$ 250,000,000	$ 70,000,000	250,000,000
Variable reference rate	one month LIBOR	one month LIBOR
Variable interest rate, basis points spread over variable reference rate (as a percent)	0.95%	1.20%
Maximum interest rate (as a percent)	2.67%	2.15%
Deferred financing costs incurred			303,000
Deferred finance costs	$ 156,000		$ 156,000	$ 187,000