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Components of Accrued Revenue (Detail) (USD $)
In Millions, unless otherwise specified
Sep. 30, 2014
Dec. 31, 2013
Sep. 30, 2013
Deferred Revenue Arrangement [Line Items]      
Accrued Revenue $ 31.4 $ 56.6 $ 40.2
Unbilled Revenues
     
Deferred Revenue Arrangement [Line Items]      
Accrued Revenue 6.8 13.0 7.2
Regulatory Asset
     
Deferred Revenue Arrangement [Line Items]      
Accrued Revenue $ 24.6 $ 43.6 $ 33.0