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Long-Term Debt - Components of Long-Term Debt (Details) - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Components of Long-term debt    
Total outstanding principal $ 700,406 $ 701,906
Less: Debt issuance costs (8,091) (8,631)
Less: Current portion (5,656) (5,656)
Long-term debt (less current portion) 686,659 687,619
Fair value of long-term debt [1] 679,425 703,572
Senior unsecured notes    
Components of Long-term debt    
Total outstanding principal 400,000 400,000
Unsecured subordinated notes    
Components of Long-term debt    
Total outstanding principal 2,656 2,656
Variable rate credit facility | Revolving credit facility    
Components of Long-term debt    
Total outstanding principal 0 0
Term Loan B, Maturing 2024    
Components of Long-term debt    
Total outstanding principal $ 297,750 $ 299,250
[1] Fair value of the Senior Notes and the term loan B were estimated based on quoted private market transactions and are classified as Level 1 in the fair value hierarchy. The fair value of the unsecured subordinated notes is determined based on a discounted cash flow analysis using current market interest rates of comparable instruments and is classified as Level 2 in the fair value hierarchy.