Columbia Environmental Project Construction (Details) (USD $)			12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012 One Year From Balance Sheet Date [Member]	Dec. 31, 2012 More Than One And Within Two Years From Balance Sheet Date [Member]	Dec. 31, 2012 Columbia Environmental Project [Member]	Dec. 31, 2012 Columbia Units [Member]
Jointly Owned Plants [Line Items]
Jointly owned plant'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%
Columbia Environmental Project [Line Items]
MGE's share of total expected capital expenditures			$ 68,000,000	$ 12,000,000	$ 140,000,000
Total incurred costs to date	98,411,000	34,055,000			59,900,000
MGE's share of AFUDC equity					$ 1,400,000
Carrying cost on pre-construction costs - AFUDC rate					100.00%