Debt - Additional Information (Details)	6 Months Ended
	Jun. 30, 2015 USD ($)	Dec. 31, 2014 USD ($)
Debt Instrument [Line Items]
Borrowings under the credit facility	$ 508,000,000	$ 415,000,000
Senior unsecured credit facility - revolving loan due 2019
Debt Instrument [Line Items]
Face value	700,000,000
Borrowings under the credit facility	508,000,000
Maximum amount available under credit facility to issue letters of credit	40,000,000
Letters of credit outstanding under credit facility	2,100,000
Undrawn availability under credit facility	$ 189,900,000
Debt instrument, covenant terms	In accordance with the terms of the new facility, we are required to maintain a minimum interest coverage ratio of 3.50 (based on the ratio of EBITDA, as defined in the credit agreement, to interest expense) and may not exceed a maximum leverage ratio of 3.50 (based on the ratio of total debt to EBITDA) throughout the term of the new facility.
Average interest rate	1.45%	1.45%
Senior unsecured credit facility - revolving loan due 2019 | Maximum
Debt Instrument [Line Items]
Interest coverage ratio required to be maintained	3.50