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Fair Value Amount of Derivative Assets Liabilities Offset in Regulatory Liabilities Assets (Detail) (Not Designated as Hedging Instrument, USD $)
In Millions, unless otherwise specified
Dec. 31, 2013
Dec. 31, 2012
Derivative Assets    
Derivative Assets $ 0.2  
Derivative Liabilities    
Derivative Liabilities   0.7
Natural Gas Contracts | Prepayments and Other
   
Derivative Assets    
Derivative Assets 0.1  
Natural Gas Contracts | Other Noncurrent Assets
   
Derivative Assets    
Derivative Assets 0.1  
Natural Gas Contracts | Other Current Liabilities
   
Derivative Liabilities    
Derivative Liabilities   $ 0.7