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Regulatory Assets (Detail) (USD $)
In Millions, unless otherwise specified
Jun. 30, 2013
Dec. 31, 2012
Jun. 30, 2012
Regulatory Assets [Line Items]      
Regulatory Assets $ 157.5 $ 186.5 $ 170.5
Less: Current Portion of Regulatory Assets 31.6 [1] 51.9 [1] 35.2 [1]
Regulatory Assets - noncurrent 125.9 134.6 135.3
Restructuring Charges | Energy Supply & Other Regulatory Tracker Mechanisms
     
Regulatory Assets [Line Items]      
Regulatory Assets 19.3 41.0 30.4
Restructuring Charges | Deferred Restructuring Costs
     
Regulatory Assets [Line Items]      
Regulatory Assets 14.5 20.1 21.3
Retirement Benefit Obligations
     
Regulatory Assets [Line Items]      
Regulatory Assets 62.5 62.5 55.1
Income Taxes
     
Regulatory Assets [Line Items]      
Regulatory Assets 9.5 10.2 10.8
Electric Utilities
     
Regulatory Assets [Line Items]      
Regulatory Assets 28.1 27.8 26.2
Other Assets
     
Regulatory Assets [Line Items]      
Regulatory Assets 6.9 8.1 9.8
Environmental Matters
     
Regulatory Assets [Line Items]      
Regulatory Assets $ 16.7 $ 16.8 $ 16.9
[1] Reflects amounts included in Accrued Revenue, discussed below, on the Company's unaudited Consolidated Balance Sheets.