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Long-Term Debt (Details) - Line of Credit
6 Months Ended
Jun. 30, 2017
USD ($)
Revolving Credit Facility  
Debt Instrument [Line Items]  
Current borrowing capacity $ 350,000,000
Remaining borrowing capacity 314,100,000
Outstanding line of credit $ 35,000,000
Automatic decrease in borrowing base 25.00%
Covenant compliance, minimum required current ratio 100.00%
Covenant compliance, maximum allowable leverage ratio 400.00%
Letter of Credit  
Debt Instrument [Line Items]  
Maximum borrowing capacity $ 15,000,000.0
Letters of credit outstanding $ 900,000