Subsequent Events Additional Information (Details) (Unsecured Debt [Member], Revolving Credit Facility [Member], USD $)	3 Months Ended	0 Months Ended
	Mar. 31, 2015	Apr. 13, 2015
Subsequent Event [Line Items]
Revolving credit facility, maximum borrowing capacity	$ 230,000,000
Interest percentage over LIBOR rate	0.875%
Subsequent Event [Member]
Subsequent Event [Line Items]
Revolving credit facility, maximum borrowing capacity		400,000,000
Interest percentage over LIBOR rate		1.00%
Letter of Credit Subfacility [Member]
Subsequent Event [Line Items]
Revolving credit facility, maximum borrowing capacity	45,000,000
Letter of credit subfacility, commission fee as a percent of face amount	0.875%
Letter of Credit Subfacility [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Revolving credit facility, maximum borrowing capacity		$ 45,000,000
Letter of credit subfacility, commission fee as a percent of face amount		1.00%