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Regulatory Assets (Detail) (USD $)
In Millions, unless otherwise specified
Sep. 30, 2013
Dec. 31, 2012
Sep. 30, 2012
Regulatory Assets [Line Items]      
Regulatory Assets $ 153.9 $ 186.5 $ 170.6
Less: Current Portion of Regulatory Assets 33.0 [1] 51.9 [1] 38.1 [1]
Regulatory Assets - noncurrent 120.9 134.6 132.5
Restructuring Charges | Energy Supply & Other Regulatory Tracker Mechanisms
     
Regulatory Assets [Line Items]      
Regulatory Assets 20.9 41.0 34.0
Restructuring Charges | Deferred Restructuring Costs
     
Regulatory Assets [Line Items]      
Regulatory Assets 11.6 20.1 20.3
Retirement Benefit Obligations
     
Regulatory Assets [Line Items]      
Regulatory Assets 62.7 62.5 55.2
Income Taxes
     
Regulatory Assets [Line Items]      
Regulatory Assets 9.3 10.2 10.6
Electric Utilities
     
Regulatory Assets [Line Items]      
Regulatory Assets 26.5 27.8 25.4
Other Assets
     
Regulatory Assets [Line Items]      
Regulatory Assets 6.8 8.1 8.4
Environmental Matters
     
Regulatory Assets [Line Items]      
Regulatory Assets $ 16.1 $ 16.8 $ 16.7
[1] Reflects amounts included in Accrued Revenue, discussed below, on the Company's unaudited Consolidated Balance Sheets.