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Regulatory Matters (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended
Dec. 31, 2011
Dec. 31, 2010
Jun. 01, 2011
2010 Rate Case [Member]
May 24, 2011
2010 Rate Case [Member]
Dec. 31, 2010
2010 Rate Case [Member]
Nov. 02, 2010
2010 Rate Case [Member]
Dec. 31, 2011
FERC-Approved Wholesale Rates [Member]
Customers
Dec. 31, 2011
Non-Affiliated Municipal Customers, Private Utilities [Member]
Sep. 30, 2011
2010 Wisconsin Rate Increase [Member]
Dec. 31, 2010
2010 Wisconsin Rate Increase [Member]
Mar. 31, 2011
DTL Reversal, Revenue Receivable Forgone [Member]
Mar. 31, 2010
DTA Tax Charge, Elimination of Medicare Part D [Member]
Jun. 30, 2011
Income Tax Benefit, MPUC Approval of Deferral of PPACA [Member]
Dec. 31, 2010
Water Rates [Member]
2010 Wisconsin Rate Increase [Member]
Dec. 31, 2010
Natural Gas Rates [Member]
2010 Wisconsin Rate Increase [Member]
Dec. 31, 2010
Electric Rates [Member]
2010 Wisconsin Rate Increase [Member]
Dec. 31, 2011
Deferred Income Tax Charges [Member]
Dec. 31, 2010
Deferred Income Tax Charges [Member]
Dec. 31, 2011
Plant Removal Obligations Costs [Member]
Dec. 31, 2010
Plant Removal Obligations Costs [Member]
Dec. 31, 2011
Other Regulatory Liabilities [Member]
Dec. 31, 2010
Other Regulatory Liabilities [Member]
Dec. 31, 2011
Deferred Fuel Costs [Member]
Dec. 31, 2010
Deferred Fuel Costs [Member]
Dec. 31, 2011
Pension Costs [Member]
Dec. 31, 2010
Pension Costs [Member]
Dec. 31, 2011
Environmental Restoration Costs [Member]
Dec. 31, 2010
Environmental Restoration Costs [Member]
Dec. 31, 2011
Deferred Income Tax Charges [Member]
Dec. 31, 2010
Deferred Income Tax Charges [Member]
Dec. 31, 2011
Asset Retirement Obligation Costs [Member]
Dec. 31, 2010
Asset Retirement Obligation Costs [Member]
Dec. 31, 2011
Medicare Part D Regulatory Asset [Member]
Dec. 31, 2010
Medicare Part D Regulatory Asset [Member]
Dec. 31, 2011
CIP Financial Incentive [Member]
Dec. 31, 2010
CIP Financial Incentive [Member]
Dec. 31, 2011
Other Regulatory Assets [Member]
Dec. 31, 2010
Other Regulatory Assets [Member]
Regulatory Matters [Line Items]                                                                            
Regulatory Assets, Current $ 17.5 $ 20.6                                         $ 17.5 [1] $ 20.6 [1]                            
Regulatory Assets, Noncurrent 345.9 310.2                                             292.8 257.9 0 20.5 28.6 17.3 9.8 7.8 5.0 0 4.6 0.7 5.1 6.0
Regulatory Assets 363.4 330.8                                                                        
Regulatory Liability, Noncurrent 43.5 43.6                             21.9 23.4 15.0 16.9 6.6 3.3                                
Approved Retail Rate Increase       53.5   53.5                                                                
Approved Rate of Return on Common Equity           10.38%       10.90%                                                        
Approved Percentage of Capital Structure Related to Equity           54.29%                                                                
MPUC, Rates, Effective Date     June 1, 2011                                                                      
Amount of Rate Differential Authorized for Collection       3.2                                                                    
Amount of Revenue Receivable Capitalized         20.5                                                                  
Tax Adjustments, Settlements, and Unusual Provisions                     6.2 4.0 2.9                                                  
Non-Affiliated Municipal Customers, Municipalities             16                                                              
Non-Affiliated Municipal Customers, Private Utilities             1                                                              
Length of Notice Required to Terminate (Years)             3 2                                                            
Approved Retail Rate Increase (Percent)                   2.40%       12.80% 2.50% 0.70%                                            
Amount Of Estimated Additional Revenue                 $ 2.0                                                          
[1] (a)Current regulatory assets are included in prepayments and other on the consolidated balance sheet.