Debt - Term Loan Facility Due Twenty Sixteen and Revolving Credit Facility - Additional Information (Details) (USD $)	0 Months Ended	12 Months Ended
	Dec. 20, 2011	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Debt Instrument [Line Items]
Principal repayment of term loan		2.50%
Principal payments on long-term debt		$ 40,000,000	$ 40,000,000	$ 20,000,000
Credit facility borrowings (Note 8)		38,000,000	28,000,000
Total interest and commitments fees		9,000,000	10,000,000	11,000,000
Amortization expense		1,000,000	1,000,000	1,000,000
Term Loan and Revolving Credit Facility
Debt Instrument [Line Items]
Maximum borrowing limits	600,000,000
Borrowings, interest rate description		interest at LIBOR plus 150 basis points, or the Eurocurrency Spread, or the alternate base rate (“ABR”) plus 50 basis points
Commitment fee on undrawn amount		0.225%
Borrowings, interest rate basis		1.70%
Term Loan and Revolving Credit Facility | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Borrowings interest rate basis point		1.50%
Term Loan and Revolving Credit Facility | Alternate Base Rate
Debt Instrument [Line Items]
Borrowings interest rate basis point		0.50%
Term Loan
Debt Instrument [Line Items]
Term Loan Facility, principal amount	400,000,000
Period of Term Loan Facility	5 years
Borrowings, maturity date	Dec. 31, 2016
Total interest and commitments fees		6,000,000	8,000,000	9,000,000
Deferred financing costs	3,500,000
Revolving Credit Facility
Debt Instrument [Line Items]
Borrowing capacity under Credit Facility	200,000,000
Credit facility, expiration period	5 years
Borrowings, maturity date	Dec. 31, 2016
Credit facility borrowings (Note 8)		0
Letter of Credit
Debt Instrument [Line Items]
Borrowing capacity under Credit Facility		40,000,000
Letters of credit outstanding amount		1,000,000
Borrowings On Same Day Notice
Debt Instrument [Line Items]
Borrowing capacity under Credit Facility		$ 40,000,000