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Regulatory Assets and Regulatory Liabilities (Detail) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2012
Dec. 31, 2011
Regulatory Assets [Line Items]    
Regulatory Assets $ 182.6 $ 185.8
Less: Current Portion of Regulatory Assets 46.6 [1] 43.6 [1]
Regulatory Assets-noncurrent 136.0 142.2
Restructuring Charges | Energy Supply Obligations
   
Regulatory Assets [Line Items]    
Regulatory Assets 8.5 16.2
Restructuring Charges | Deferred Restructuring Costs
   
Regulatory Assets [Line Items]    
Regulatory Assets 20.1 21.8
Retirement Benefit Obligations
   
Regulatory Assets [Line Items]    
Regulatory Assets 62.5 55.3
Income Taxes
   
Regulatory Assets [Line Items]    
Regulatory Assets 10.2 10.9
Electric Utilities
   
Regulatory Assets [Line Items]    
Regulatory Assets 27.8 28.6
Regulatory Tracker Mechanisms
   
Regulatory Assets [Line Items]    
Regulatory Assets 24.7 23.5
Other Assets
   
Regulatory Assets [Line Items]    
Regulatory Assets 12.0 12.0
Environmental Matters
   
Regulatory Assets [Line Items]    
Regulatory Assets $ 16.8 $ 17.5
[1] Reflects amounts included in Accrued Revenue on the Company's Consolidated Balance Sheets.