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Note 6 - Derivative Instruments (Details Textual)
gal in Millions, $ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2017
USD ($)
$ / gal
Jun. 30, 2016
$ / gal
Jun. 30, 2017
USD ($)
gal
Aug. 31, 2015
USD ($)
Cash Flow Hedge Derivative Instrument Assets at Fair Value $ 5.4   $ 5.4  
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net $ 1.5   2.7  
Cash Flow Hedge Gain (Loss) to be Reclassified within Twelve Months     2.6  
Maximum [Member]        
Underlying, Derivative Volume | $ / gal 1.62 1.53    
Minimum [Member]        
Underlying, Derivative Volume | $ / gal 1.33 1    
Interest Rate Swap [Member]        
Derivative, Notional Amount       $ 28.0
Derivative, Fixed Interest Rate       4.20%
Derivative Instruments in Hedges, Liabilities, at Fair Value $ 0.7   0.7  
Derivative Instruments, Gain (Loss) Reclassification from Accumulated OCI to Income, Estimated Net Amount to be Transferred     0.2  
Interest Rate Swap [Member] | Interest Expense [Member]        
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net     $ 0.1  
Fuel Hedge Contracts for 2017 Fuel Requirements [Member]        
Derivative, Nonmonetary Notional Amount, Volume | gal     6.0  
Percent of Projected Fuel Requirements     24.60%  
Fuel Hedge Contracts for 2018 Requirements [Member]        
Derivative, Nonmonetary Notional Amount, Volume | gal     7.6  
Percent of Projected Fuel Requirements     15.40%