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Summary of Interest Expense and Interest Income (Detail) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Interest Expense      
Long-term Debt $ 20.3 $ 20.3 $ 20.0
Short-term Debt 1.5 1.7 1.7
Interest Expense 21.8 21.8 22.1
Interest Expense 22.3 22.0 22.0
Interest Income      
Interest Income (4.2) (1.6) (3.9)
Total Interest Expense, net 18.1 20.4 18.1
Other Regulatory Liability
      
Interest Expense      
Interest Expense 0.5   0.3
Regulatory Asset
      
Interest Income      
Interest Income (3.4) (1.1) (3.5)
Allowance for Funds Used During Construction, Capitalized Interest
      
Interest Income      
Interest Income $ (0.8) [1] $ (0.5) [1] $ (0.4) [1]
[1] AFUDC-Allowance for Funds Used During Construction