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Debt - 7.75% Senior Secured Notes Due 2025 (Details) - USD ($)
$ in Thousands
Dec. 21, 2017
Jun. 30, 2019
Dec. 31, 2018
Debt Instrument [Line Items]      
Principal amount of long-term debt   $ 672,973 $ 416,466
7.75% Senior Secured Note due 2025      
Debt Instrument [Line Items]      
Debt instrument, interest rate   7.75%  
Senior Notes | 7.75% Senior Secured Note due 2025      
Debt Instrument [Line Items]      
Principal amount of long-term debt $ 300,000 $ 300,000 $ 300,000
Proceeds from issuance of senior long-term debt $ 289,200    
Unamortized discount (premium) and debt issuance costs, percentage 1.00%    
Debt instrument, interest rate 7.75% 7.75%