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Fair Value Measurements (Details) - USD ($)
$ in Thousands
Dec. 31, 2018
Dec. 31, 2017
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Debt instrument, carrying value $ 518,711 $ 507,906
Senior secured revolving credit facility    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Debt instrument, carrying value 119,000 108,446
Senior unsecured notes due October 2021    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Debt instrument, carrying value 400,000 400,000
Senior unsecured notes due October 2021 | Significant other observable inputs (Level 2)    
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]    
Debt instrument, carrying value 398,100 397,400
Debt instrument, fair value $ 362,000 $ 402,000