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Regulatory Matters (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended 3 Months Ended
Mar. 31, 2012
Dec. 31, 2011
Mar. 31, 2012
Medicare Part D [Member]
Dec. 31, 2011
Medicare Part D [Member]
Jun. 30, 2011
Income Tax Benefit, MPUC Approval of Deferral of PPACA [Member]
Mar. 31, 2012
Income Tax Benefit, MPUC Approval of Deferral of PPACA [Member]
Medicare Part D [Member]
Mar. 31, 2010
DTA Reversal, Elimination of Medicare Part D [Member]
Jun. 01, 2011
2010 MN Rate Case [Member]
Mar. 31, 2012
FERC-Approved Wholesale Rates [Member]
Customers
Jun. 01, 2011
FERC-Approved Wholesale Rates [Member]
Mar. 31, 2012
Non-Affiliated Municipal Customers, Private Utilities [Member]
Dec. 31, 2010
2010 Wisconsin Rate Increase [Member]
Electric Rates [Abstract]                        
Approved Rate of Return on Common Equity                   10.38%   10.90%
Approved Percentage of Capital Structure Related to Equity               54.29%        
Tax Adjustments, Settlements, and Unusual Provisions         $ 2.9   $ 4.0          
Non-Affiliated Municipal Customers, Municipalities                 16      
Non-Affiliated Municipal Customers, Private Utilities                 1      
Length of Notice Required to Terminate (Years)                 3   2  
The Patient Protection and Affordable Care Act of 2010 (PPACA) [Abstract]                        
Tax Adjustments, Settlements, and Unusual Provisions         2.9   4.0          
Non-Current Regulatory Assets $ 339.3 $ 345.9 $ 5.0 $ 5.0   $ 5.0