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Note 6 - Derivative Instruments (Details Textual)
gal in Millions, $ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2018
USD ($)
$ / gal
Jun. 30, 2017
$ / gal
Jun. 30, 2018
USD ($)
gal
Aug. 31, 2015
USD ($)
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net, Total $ 0.5   $ 0.7  
Cash Flow Hedge Derivative Instrument Assets at Fair Value $ 1.3   1.3  
Cash Flow Hedge Gain (Loss) to be Reclassified within Twelve Months     0.9  
Maximum [Member]        
Underlying, Derivative Volume | $ / gal 2.25 1.62    
Minimum [Member]        
Underlying, Derivative Volume | $ / gal 1.92 1.33    
Interest Rate Swap [Member]        
Derivative, Notional Amount       $ 28.0
Derivative, Fixed Interest Rate       4.20%
Derivative Instruments in Hedges, Liabilities, at Fair Value, Total $ 0.9   0.9  
Derivative Instruments, Gain (Loss) Reclassification from Accumulated OCI to Income, Estimated Net Amount to be Transferred     $ 0.1  
Interest Rate Swap [Member] | Interest Expense [Member]        
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net, Total $ 0.1      
Fuel Hedge Contracts for 2018 Requirements [Member]        
Derivative, Nonmonetary Notional Amount, Volume | gal     3.8  
Percent of Projected Fuel Requirements     16.90%