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Components of Accrued Revenue (Detail) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2013
Dec. 31, 2012
Deferred Revenue Arrangement [Line Items]    
Accrued Revenue $ 56.6 $ 63.4
Regulatory Asset
   
Deferred Revenue Arrangement [Line Items]    
Accrued Revenue 43.6 51.9
Unbilled Revenues
   
Deferred Revenue Arrangement [Line Items]    
Accrued Revenue $ 13.0 $ 11.5