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Regulatory Assets (Detail) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2013
Dec. 31, 2012
Regulatory Assets [Line Items]    
Regulatory Assets $ 143.7 $ 186.5
Less: Current Portion of Regulatory Assets 43.6 [1] 51.9 [1]
Regulatory Assets-noncurrent 100.1 134.6
Restructuring Charges | Energy Supply & Other Regulatory Tracker Mechanisms
    
Regulatory Assets [Line Items]    
Regulatory Assets 32.5 41.0
Restructuring Charges | Deferred Restructuring Costs
    
Regulatory Assets [Line Items]    
Regulatory Assets 9.3 20.1
Retirement Benefits
    
Regulatory Assets [Line Items]    
Regulatory Assets 42.6 62.5
Income Taxes
    
Regulatory Assets [Line Items]    
Regulatory Assets 11.9 10.2
Electric Utilities
    
Regulatory Assets [Line Items]    
Regulatory Assets 25.6 27.8
Other Assets
    
Regulatory Assets [Line Items]    
Regulatory Assets 5.7 8.1
Environmental Matters
    
Regulatory Assets [Line Items]    
Regulatory Assets $ 16.1 $ 16.8
[1] Reflects amounts included in Accrued Revenue on the Company's Consolidated Balance Sheets.