Elm Road (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012
Elm Road Units ownership [Line Items]
Elm Road Unit costs prudently incurred	100.00%
Approved recovery of the Elm Road Unit costs	99.50%
Deferred recovery of the Elm Road Unit costs	$ 1.8
MGE Power Elm Road [Member] | Elm Road Units [Member]
Elm Road Units ownership [Line Items]
Ownership interest in jointly owned plant	8.33%
Plant capacity - MW	615