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Fair Value Measurements (Narrative) (Details)
gal in Millions
1 Months Ended 3 Months Ended
May 31, 2017
USD ($)
Mar. 31, 2020
USD ($)
$ / gal
gal
Mar. 31, 2019
USD ($)
Dec. 31, 2019
USD ($)
Derivatives Fair Value [Line Items]        
Derivative underlying hedge percent   100.00%    
Derivative, Nonmonetary Notional Amount, Volume | gal   11.4    
Fair value hedge liability   $ 8,695,000    
Fair value hedge assets       $ 849,000
Reclassification of derivative (gains) losses to earnings—net of tax   1,338,000 $ 891,000  
8.000% Senior Notes Due in 2022 [Member]        
Derivatives Fair Value [Line Items]        
Debt instrument, face amount $ 325,000,000      
Stated interest rate 8.00%      
Maturity date May 15, 2022      
8.000% Senior Notes Due in 2022 [Member] | Fair Value, Inputs, Level 1 [Member]        
Derivatives Fair Value [Line Items]        
Fair value of debt   $ 318,094,000    
Minimum [Member]        
Derivatives Fair Value [Line Items]        
Fixed price range | $ / gal   1.12    
Maximum [Member]        
Derivatives Fair Value [Line Items]        
Fixed price range | $ / gal   2.05    
Fuel Hedge Contracts [Member]        
Derivatives Fair Value [Line Items]        
Derivative underlying hedge percent   80.00%    
Derivative underlying hedge adjusted, percent   100.00%