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Income Tax Expense - Reconciliation of Taxes from Federal Statutory Rate to Total Income Tax Expense (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended 12 Months Ended
Jun. 30, 2011
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Income Tax Expense [Abstract]        
Income Before Non-Controlling Interest and Income Taxes   $ 135.1 $ 129.2 $ 119.1
Statutory Federal Income Tax Rate   35.00% 35.00% 35.00%
Income Taxes Computed at 35 Percent Statutory Federal Rate   47.3 45.2 41.7
Increase (Decrease) in Tax [Abstract]        
State Income Taxes - Net of Federal Income Tax Benefit   1.2 6.0 4.5
Unusual or Infrequent Item [Line Items]        
Deferred Income Tax Expense (Benefit)   37.5 35.8 66.0
Regulatory Differences for Utility Plant   (2.2) (1.2) (2.0)
Production Tax Credits   7.6 4.3 1.6
Other   (0.7) (1.0) (2.3)
Income Tax Expense   38.0 35.6 44.3
Effective Income Tax Rate   28.10% 27.60% 37.20%
PPACA Elimination of Medicare Part D [Member]
        
Unusual or Infrequent Item [Line Items]        
Tax Adjustments, Settlements, and Unusual Provisions   0 0 4.0
Deferred Income Tax Expense (Benefit) 2.9 0 (2.9) 0
2010 Rate Case Stipulation Agreement [Member]
        
Unusual or Infrequent Item [Line Items]        
Deferred Income Tax Expense (Benefit)   $ 0 $ (6.2) $ 0