Columbia Environmental Project Construction (Details) (USD $)			9 Months Ended
	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2012 Columbia Environmental Project [Member]	Sep. 30, 2012 Columbia Environmental Project [Member] One Year From Balance Sheet Date [Member]	Sep. 30, 2012 Columbia Environmental Project [Member] More Than One And Within Two Years From Balance Sheet Date [Member]	Sep. 30, 2012 Columbia Environmental Project [Member] More Than Two And Within Three Years From Balance Sheet Date [Member]	Sep. 30, 2012 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%
Expected Capital Expenditures [Line Items]
MGE's share of expected capital expenditures			$ 140,000,000	$ 18,000,000	$ 68,000,000	$ 12,000,000
Total incurred costs to date	75,748,000	34,055,000	42,300,000
Carrying cost on pre-construction costs - AFUDC rate			100.00%
Columbia AFUDC Equity			$ 700,000