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Debt - Summary of Amendment to Senior Secured Revolving Credit Facility (Descriptors) (Details)
 12 Months Ended
Dec. 31, 2017
USD ($)
Senior Secured Notes [Line Items]  
Amounts which can be increased to the credit facility, subject to lender approval (up to) $ 150,000,000
Minimum  
Senior Secured Notes [Line Items]  
Line of credit facility commitment fee, basis points (in percentage) 0.20%
Minimum | LIBOR  
Senior Secured Notes [Line Items]  
Line of credit, interest spread basis points (in percentage) 1.55%
Maximum  
Senior Secured Notes [Line Items]  
Line of credit facility commitment fee, basis points (in percentage) 0.30%
Maximum | LIBOR  
Senior Secured Notes [Line Items]  
Line of credit, interest spread basis points (in percentage) 2.30%