Columbia Environmental Project Construction (Details) (USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Expected Capital Expenditures [Line Items]
Total incurred costs to date	$ 115,803,000		98,411,000
MGE rate of return	50.00%
Columbia Environmental Project [Member]
Expected Capital Expenditures [Line Items]
MGE's share of expected capital expenditures	140,000,000
Total incurred costs to date	78,800,000
Carrying cost on pre-construction costs - AFUDC rate	50.00%		100.00%
AFUDC - equity funds	500,000	100,000
Columbia Environmental Project [Member] | One Year From Balance Sheet Date [Member]
Expected Capital Expenditures [Line Items]
MGE's share of expected capital expenditures	49,000,000
Columbia Environmental Project [Member] | More Than One And Within Two Years From Balance Sheet Date [Member]
Expected Capital Expenditures [Line Items]
MGE's share of expected capital expenditures	$ 12,000,000
Columbia Units [Member]
Columbia Units [Line Items]
Columbia's share of MGE's net generating capability	29.00%
Ownership interest in jointly owned plant	22.00%
WPL ownership interest of Columbia	46.20%
WPSC ownership interest of Columbia	31.80%