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Note 13 - Derivative Instruments (Details Textual)
gal in Millions, $ in Millions
12 Months Ended
Dec. 31, 2017
USD ($)
$ / item
gal
Dec. 31, 2016
USD ($)
$ / item
Dec. 31, 2015
USD ($)
Aug. 31, 2015
USD ($)
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net $ 4.1 $ 16.7 $ 15.3  
Cash Flow Hedge Derivative Instrument Assets at Fair Value 0.8 $ 3.6    
Cash Flow Hedge Gain (Loss) to be Reclassified within Twelve Months $ 0.6      
Maximum [Member]        
Underlying, Derivative Volume | $ / item 1.97 1.66    
Minimum [Member]        
Underlying, Derivative Volume | $ / item 1.33 0.83    
Interest Rate Swap [Member]        
Derivative, Notional Amount       $ 28.0
Derivative, Fixed Interest Rate       4.20%
Derivative Instruments in Hedges, Liabilities, at Fair Value $ 0.4 $ 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 $ 0.4 $ 0.6    
Fuel Hedge Contracts for 2018 Requirements [Member]        
Derivative, Nonmonetary Notional Amount, Volume | gal 7.6      
Percent of Projected Fuel Requirements 16.10%      
Additional Fuel Expense No Longer Deemed to be Effective [Member]        
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net     1.4  
Hedge Contracts, Ineffectiveness [Member]        
Derivative Reduction of Fuel Expense     $ 1.4