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Regulatory Assets (Detail) (USD $)
In Millions, unless otherwise specified
Jun. 30, 2014
Dec. 31, 2013
Jun. 30, 2013
Regulatory Assets [Line Items]      
Regulatory Assets $ 115.4 $ 143.7 $ 157.5
Less: Current Portion of Regulatory Assets 27.3 [1] 43.6 [1] 31.6 [1]
Regulatory Assets - noncurrent 88.1 100.1 125.9
Environmental Matters
      
Regulatory Assets [Line Items]      
Regulatory Assets 10.6 16.1 16.7
Retirement Benefit
      
Regulatory Assets [Line Items]      
Regulatory Assets 42.2 42.6 62.5
Income Taxes
      
Regulatory Assets [Line Items]      
Regulatory Assets 10.4 11.9 9.5
Electric Utilities
      
Regulatory Assets [Line Items]      
Regulatory Assets 22.3 25.6 28.1
Other Assets
      
Regulatory Assets [Line Items]      
Regulatory Assets 7.5 5.7 6.9
Energy Supply & Other Regulatory Tracker Mechanisms
      
Regulatory Assets [Line Items]      
Regulatory Assets 17.7 32.5 19.3
Deferred Restructuring Costs
      
Regulatory Assets [Line Items]      
Regulatory Assets $ 4.7 $ 9.3 $ 14.5
[1] Reflects amounts included in Accrued Revenue, discussed above, on the Company's Consolidated Balance Sheets.