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Credit Facilities and Long-Term Debt, Revolving Credit Facility (Details) - USD ($)
$ in Thousands
9 Months Ended 12 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Fixed Assets Not Included in Borrowing Base [Member] | Maximum [Member]      
Line of Credit Facility [Abstract]      
Borrowing base $ 25,000    
Borrowing base percentage 10.00%    
Fixed Assets Included in Borrowing Base [Member]      
Line of Credit Facility [Abstract]      
Borrowing base $ 31,250    
Borrowing base percentage 12.50%    
JPMorgan Chase Bank Credit Agreement [Member]      
Line of Credit Facility [Abstract]      
Maximum borrowing capacity $ 250,000    
Line of credit facility, additional borrowing capacity $ 50,000    
Maturity date Oct. 31, 2020    
Additional available borrowing capacity $ 201,800    
Outstanding borrowings under credit facility $ 45,000   $ 57,000
Weighted average interest rate 3.40%   2.70%
JPMorgan Chase Bank Credit Agreement [Member] | LIBOR [Member] | Minimum [Member]      
Line of Credit Facility [Abstract]      
Margin on variable rate 1.25%    
JPMorgan Chase Bank Credit Agreement [Member] | LIBOR [Member] | Maximum [Member]      
Line of Credit Facility [Abstract]      
Margin on variable rate 1.75%    
JPMorgan Chase Bank Credit Agreement [Member] | Alternate Base Rate [Member]      
Line of Credit Facility [Abstract]      
Average daily loan balance outstanding $ 2,000 $ 4,600 $ 3,800
JPMorgan Chase Bank Credit Agreement [Member] | Alternate Base Rate [Member] | Minimum [Member]      
Line of Credit Facility [Abstract]      
Margin on variable rate 0.25%    
JPMorgan Chase Bank Credit Agreement [Member] | Alternate Base Rate [Member] | Maximum [Member]      
Line of Credit Facility [Abstract]      
Margin on variable rate 0.75%    
JPMorgan Chase Bank Credit Agreement [Member] | Direct Borrowings [Member]      
Line of Credit Facility [Abstract]      
Outstanding borrowings under credit facility $ 45,000   $ 57,000
Line of Credit [Member]      
Line of Credit Facility [Abstract]      
Coverage ratio 1    
Line of Credit [Member] | Pay Cash Dividend [Member]      
Line of Credit Facility [Abstract]      
Agreement permissions $ 20,000    
Line of Credit [Member] | Stock Repurchase [Member]      
Line of Credit Facility [Abstract]      
Agreement permissions 20,000    
Line of Credit [Member] | Stock Repurchase [Member] | Minimum [Member]      
Line of Credit Facility [Abstract]      
Borrowing base 25,000    
Line of Credit [Member] | The Credit Agreement Also Permits Acquisitions, Permissible Debt, Cash Dividend Payments And Stock Repurchases [Member] | Minimum [Member]      
Line of Credit Facility [Abstract]      
Agreement permissions 20,000    
Line of Credit [Member] | Canada [Member]      
Line of Credit Facility [Abstract]      
Maximum borrowing capacity $ 10,000