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Energy Supply Obligations (Detail) (USD $)
In Millions, unless otherwise specified
Sep. 30, 2014
Dec. 31, 2013
Sep. 30, 2013
Contractual Obligation [Line Items]      
Energy Supply Obligations $ 18.1 $ 14.4 $ 15.4
Total Energy Supply Obligations 20.2 16.9 18.1
Exchange Gas Obligation
     
Contractual Obligation [Line Items]      
Energy Supply Obligations 13.4 9.8 11.8
Renewable Energy Portfolio Standards
     
Contractual Obligation [Line Items]      
Energy Supply Obligations 4.1 3.7 2.7
Power Supply Contract Divestitures
     
Contractual Obligation [Line Items]      
Energy Supply Obligations 0.6 0.9 0.9
Energy Supply Obligations $ 2.1 $ 2.5 $ 2.7