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Activity in Company's Allowance for Doubtful Accounts (Detail) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2013
Electric
Dec. 31, 2012
Electric
Dec. 31, 2011
Electric
Dec. 31, 2013
Gas
Dec. 31, 2012
Gas
Dec. 31, 2011
Gas
Dec. 31, 2013
Other
Dec. 31, 2012
Other
Dec. 31, 2011
Other
Jan. 01, 2011
Other
Accounts and Other Receivables [Line Items]                          
Balance at Beginning of Period $ 1.9 $ 2.3 $ 2.6 $ 1.1 $ 1.7 $ 1.8 $ 0.7 $ 0.5 $ 0.7 $ 0.1 $ 0.1 $ 0.1 $ 0.1
Provision 4.6 3.6 4.3 2.6 1.4 2.1 2.0 2.2 2.2        
Recoveries 0.4 0.6 0.5 0.2 0.3 0.2 0.2 0.3 0.3        
Accounts Written Off 5.3 4.6 5.1 2.6 2.3 2.4 2.7 2.3 2.7        
Balance at End of Period $ 1.6 $ 1.9 $ 2.3 $ 1.3 $ 1.1 $ 1.7 $ 0.2 $ 0.7 $ 0.5 $ 0.1 $ 0.1 $ 0.1 $ 0.1