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Equity and Financing Arrangements (Details) (USD $)
In Millions, except Share data, unless otherwise specified
6 Months Ended 3 Months Ended
Jun. 30, 2013
Jun. 30, 2012
Jul. 18, 2013
Issuance of debt - 4.42%, due 2043 [Member]
Jul. 18, 2013
Issuance of debt - 4.47%, due 2048 [Member]
Jul. 18, 2013
Repayment of debt - 5.26%, due 2017 [Member]
Jul. 18, 2013
Repayment of debt - 5.59%, due 2018 [Member]
Jul. 18, 2013
5.59%, due 2018 [Member]
Jun. 30, 2013
MGE Energy [Member]
Jun. 19, 2013
MGE Energy [Member]
Jun. 30, 2013
MGE [Member]
Jun. 19, 2013
MGE [Member]
Common Stock Disclosures [Abstract]                      
Common stock issued during period 0 0                  
Dilutive Securities [Abstract]                      
Dilutive Securities $ 0                    
Credit Facilities [Line Items]                      
Revolving credit facility               50.0 40.0 100.0 75.0
Floating rate-based borrowings adder minimum               0.00%   0.00%  
Floating rate-based borrowings adder maximum               0.125%   0.125%  
Eurodollar-based borrowings adder minimum               0.675%   0.675%  
Eurodollar-based borrowings adder maximum               1.125%   1.125%  
Federal funds fixed base rate               0.50%   0.50%  
Eurodollar fixed base rate               1.00%   1.00%  
Line of credit borrowings               0   0  
Commercial paper outstanding                   14.0  
Debt Instruments [Line Items]                      
Unsecured debt     20.0 20.0              
Interest rate     4.42% 4.47% 5.26% 5.59% 5.59%        
Redeemed and retired debt         20.0 20.0          
Make-whole premium         3.2 3.6          
Secured debt             $ 20.0