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Fair Value Offset in Regulatory Assets (Detail) (Not Designated as Hedging Instrument, USD $)
In Millions, unless otherwise specified
Dec. 31, 2012
Dec. 31, 2011
Regulatory Assets [Line Items]    
Natural Gas Futures Contracts $ 0.7 [1] $ 2.3 [1]
Natural Gas Contracts | Other Current Liabilities
   
Regulatory Assets [Line Items]    
Natural Gas Futures Contracts 0.7 [1] 1.7 [1]
Natural Gas Contracts | Other Noncurrent Liabilities
   
Regulatory Assets [Line Items]    
Natural Gas Futures Contracts   $ 0.6 [1]
[1] The current portion of Regulatory Assets is recorded as Accrued Revenue on the Company's Consolidated Balance Sheets.