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Note 6 - Derivative Instruments (Detail) (USD $)
In Millions, unless otherwise specified
 3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2012
Dec. 31, 2012
Jun. 30, 2012
Dec. 31, 2013
Derivative, Number of Instruments Held 12,100,000   12,100,000  
Percent of Projected Fuel Requirements   20.00%   22.00%
Derivative Liability, Fair Value, Net $ 3.3   $ 3.3  
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net 1.0   1.8  
Gain (Loss) on Cash Flow Hedge Ineffectiveness, Net 0.4   0.2  
Cash Flow Hedge Gain (Loss) to be Reclassified within Twelve Months $ 1.3      
Gallons for Remainder of 2012 [Member]
        
Derivative, Number of Instruments Held 6,000,000   6,000,000