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Debt - Additional Information (Details)
1 Months Ended 3 Months Ended 9 Months Ended 12 Months Ended
Aug. 31, 2015
USD ($)
Sep. 30, 2015
USD ($)
Sep. 30, 2015
USD ($)
Dec. 31, 2014
USD ($)
Sep. 30, 2014
USD ($)
Debt Instrument [Line Items]          
Borrowings under the credit facility   $ 596,400,000 $ 596,400,000 $ 415,000,000  
4.7% senior notes due 2025          
Debt Instrument [Line Items]          
Face value $ 300,000,000        
Debt instrument, interest rate 4.70% 4.70% 4.70%    
Debt instrument, maturity year 2025   2025    
Increase in senior notes interest rate 0.25%        
Effective interest rate   4.76% 4.76%    
Unamortized deferred financing costs and debt discount   $ 3,600,000 $ 3,600,000    
4.7% senior notes due 2025 | Level 2          
Debt Instrument [Line Items]          
Fair value of senior notes   303,000,000 303,000,000    
Senior unsecured credit facility - revolving loan due 2019          
Debt Instrument [Line Items]          
Face value   700,000,000 700,000,000   $ 700,000,000
Repayments of debt $ 296,400,000        
Borrowings under the credit facility   300,000,000 300,000,000    
Maximum amount available under credit facility to issue letters of credit   40,000,000 40,000,000    
Letters of credit outstanding under credit facility   2,100,000 2,100,000    
Undrawn availability under credit facility   $ 397,900,000 $ 397,900,000    
Debt instrument, covenant terms     In accordance with the terms of the 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 Facility.    
Average interest rate   2.00% 1.84% 1.88%  
Maximum | 4.7% senior notes due 2025          
Debt Instrument [Line Items]          
Debt instrument, interest rate 6.70%        
Maximum | Senior unsecured credit facility - revolving loan due 2019          
Debt Instrument [Line Items]          
Interest coverage ratio required to be maintained     3.50