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Credit Arrangement (Details)
$ in Millions
6 Months Ended
Jun. 30, 2018
USD ($)
Credit Agreement [Member]  
Line of Credit Facility, Amount [Abstract]  
Maximum borrowing capacity $ 60.0
Reduction in borrowing base due to deficit in fixed charges coverage ratio test $ 6.0
Number of trailing months to compute fixed charges coverage ratio 12 months
Current borrowing base $ 54.4
Net availability $ 48.4
Maturity date Mar. 31, 2021
Letter of Credit [Member]  
Line of Credit Facility, Amount [Abstract]  
Outstanding letters of credit $ 0.0