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Long-term Debt - Components of Long-term Debt (Details) - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Jun. 30, 2017
Dec. 31, 2016
Debt Instrument [Line Items]        
Less: Amounts due within one year $ 14,556 $ 27,870    
Long-term debt, gross 676,572 485,261    
Less: Deferred debt issuance costs 10,372 4,202 $ 5,042 $ 5,892
Long-term debt 666,200 481,059    
Senior Notes        
Debt Instrument [Line Items]        
Long-term debt, gross, including current maturities 400,000 0    
Credit Facility | New Credit Facility        
Debt Instrument [Line Items]        
Long-term debt, gross, including current maturities 291,128 0    
Credit Facility | Former Credit Facility        
Debt Instrument [Line Items]        
Long-term debt, gross, including current maturities $ 0 $ 513,131