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Fair Value Measurements - Schedule of Senior Secured Credit Facility (Details) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Debt Instrument [Line Items]    
Transfer of asset fair value Level 1 to 2 $ 0  
Transfer of liability fair value Level 1 to 2 0  
Transfer of asset fair value Level 2 to 1 0  
Transfer of liability fair value Level 2 to 1 0  
Transfers of assets or liabilities between the fair value measurement levels 3 0  
Carrying amounts | 2016 Senior Secured Credit Facility    
Debt Instrument [Line Items]    
Long term debt, carrying amount 228,986,000 $ 230,820,000
Level 2 | Estimated fair value | 2016 Senior Secured Credit Facility    
Debt Instrument [Line Items]    
Long term debt, fair value $ 232,933,000 $ 233,240,000