Commitments and Contingencies - Additional Information (Details) - Unsecured Debt [Member] - Credit Facility [Member] - USD ($)	6 Months Ended
	Jun. 30, 2015	May. 01, 2015
Credit Facility [Line Items]
Credit facility, maximum borrowing capacity		$ 500,000,000
Credit facility, expiration date	Mar. 01, 2018
Average Quarterly Utilization of Facility is Less than 20% [Member]
Credit Facility [Line Items]
Percentage fee paid for undrawn portion of credit facility	0.125%
Average Quarterly Utilization of Facility is 20% or More [Member]
Credit Facility [Line Items]
Percentage fee paid for undrawn portion of credit facility	0.10%
London Interbank Offered Rate (LIBOR) [Member]
Credit Facility [Line Items]
Interest percentage over LIBOR rate	0.95%
Revolving Credit Facility [Member]
Credit Facility [Line Items]
Effective interest rate during period on outstanding line of credit	1.10%
Letter of Credit Subfacility [Member]
Credit Facility [Line Items]
Credit facility, maximum borrowing capacity		$ 55,000,000