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Energy Supply Obligations (Detail) (USD $)
In Millions, unless otherwise specified
Jun. 30, 2014
Dec. 31, 2013
Jun. 30, 2013
Contractual Obligation [Line Items]      
Energy Supply Obligations $ 12.3 $ 14.4 $ 9.8
Total Energy Supply Obligations 14.5 16.9 12.7
Exchange Gas Obligation
     
Contractual Obligation [Line Items]      
Energy Supply Obligations 7.6 9.8 7.0
Renewable Energy Portfolio Standards
     
Contractual Obligation [Line Items]      
Energy Supply Obligations 4.0 3.7 1.9
Power Supply Contract Divestitures
     
Contractual Obligation [Line Items]      
Energy Supply Obligations 0.7 0.9 0.9
Energy Supply Obligations $ 2.2 $ 2.5 $ 2.9