Subsequent Events (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Subsequent Events [Line Items]
Contributions to ATC	$ 1,008	$ 810	$ 3,701
MGE Transco [Member] \| ATC [Member]
Subsequent Events [Line Items]
Contributions to ATC	900	700	3,600
MGE Transco [Member] \| ATC [Member] \| ATC Capital Contribution [Member]
Subsequent Events [Line Items]
Subsequent Event, Description	On January 30, 2012, MGE Transco made a voluntary $0.4 million capital contribution to ATC.
Contributions to ATC	$ 400

Describes the event or transaction that occurred between the balance sheet date and the date the financial statements are issued or available to be issued.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 5

-Paragraph 11

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 855

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=6842918&loc=SL6314017-165662

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 165

-Paragraph 13

-Subparagraph a

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration