Columbia Environmental Project Construction (Details) (USD $)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Expected Capital Expenditures [Line Items]
Total incurred costs to date	$ 131,096,000		$ 131,096,000		$ 98,411,000
Authorized MGE rate of return	50.00%		50.00%
Columbia Environmental Project [Member]
Expected Capital Expenditures [Line Items]
MGE's total share of expected capital expenditures			140,000,000
Total incurred costs to date	113,600,000		113,600,000
MGE's share of expected capital expenditures current year			12,000,000
MGE's share of expected capital expenditures next year			14,000,000
Columbia environmental project commitments	24,400,000		24,400,000
AFUDC - equity funds	$ 800,000	$ 400,000	$ 1,900,000	$ 700,000
Authorized AFUDC Rate			50.00%		100.00%
Columbia Units [Member]
Jointly Owned Utility Plant Interests [Line Items]
WPL ownership interest of Columbia	46.20%		46.20%
WPSC ownership interest of Columbia	31.80%		31.80%
MGE ownership interest in jointly owned plant	22.00%		22.00%