Common Equity (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Common Equity [Line Items]
Common stock dividends declared									$ 35,026	[1]	$ 34,370	[2]	$ 33,693	[3]
Common stock dividends declared, per share	$ 0.383	$ 0.383	$ 0.375	$ 0.375	$ 0.375	$ 0.375	$ 0.368	$ 0.368	$ 1.52		$ 1.49		$ 1.46
Shares of preferred stock	1,175,000								1,175,000
Par value of preferred stock	$ 25								$ 25
MGE [Member]
Common Equity [Line Items]
Cash dividends paid to parent by MGE									$ 26,648		$ 26,150		$ 19,318

[1] $1.52 per share

[2] $1.49 per share

[3] $1.46 per share