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Fair Value Offset in Regulatory Assets (Detail) (Not Designated as Hedging Instrument, USD $)
In Millions, unless otherwise specified
Sep. 30, 2012
Dec. 31, 2011
Sep. 30, 2011
Regulatory Assets [Line Items]      
Natural Gas Futures Contracts $ 1.2 [1] $ 2.3 [1] $ 1.5 [1]
Natural Gas Contracts | Other Current Liabilities
      
Regulatory Assets [Line Items]      
Natural Gas Futures Contracts 1.0 [1] 1.7 [1] 1.2 [1]
Natural Gas Contracts | Other Noncurrent Liabilities
      
Regulatory Assets [Line Items]      
Natural Gas Futures Contracts $ 0.2 [1] $ 0.6 [1] $ 0.3 [1]
[1] The current portion of Regulatory Assets is recorded as Accrued Revenue on the Company's unaudited Consolidated Balance Sheets.