Variable Interest Entities (Details)		Dec. 31, 2019 USD ($) MW	Dec. 31, 2018 USD ($) MW
Significant Balance Sheet Accounts Related to Interests in Variable Interest Entities
Property, plant, and equipment, net		$ 1,530,199,000	$ 1,369,766,000
Construction work in progress		112,484,000	139,671,000
Accrued interest and accrued (prepaid) taxes		7,244,000	7,384,000
Deferred income taxes		243,302,000	231,952,000
Unamortized discount and debt issuance costs, net		$ 4,479,000	$ 4,535,000
Other Variable Interest Entities
Other variable interest entities, purchase power agreement, plant capacity (in MW) | MW		13	11
MGE Power Elm Road [Member]
Significant Balance Sheet Accounts Related to Interests in Variable Interest Entities
Property, plant, and equipment, net		$ 170,763,000	$ 173,464,000
Construction work in progress		468,000	530,000
Affiliate receivables, current and noncurrent		0	0
Accrued interest and accrued (prepaid) taxes		1,364,000	533,000
Deferred income taxes		30,621,000	30,595,000
Long-term debt	[1]	54,207,000	56,806,000
Noncontrolling interest	[2]	97,172,000	97,519,000
Unamortized discount and debt issuance costs, net		400,000	500,000
MGE Power West Campus [Member]
Significant Balance Sheet Accounts Related to Interests in Variable Interest Entities
Property, plant, and equipment, net		79,473,000	81,273,000
Construction work in progress		309,000	478,000
Affiliate receivables, current and noncurrent		3,600,000	3,707,000
Accrued interest and accrued (prepaid) taxes		(160,000)	(23,000)
Deferred income taxes		14,363,000	14,222,000
Long-term debt	[1]	39,627,000	41,496,000
Noncontrolling interest	[2]	43,131,000	43,935,000
Unamortized discount and debt issuance costs, net		$ 100,000	$ 100,000

[1] MGE Power Elm Road's long-term debt includes debt issuance costs of $0.4 million and $0.5 million as of December 31, 2019 and 2018, respectively. The debt is secured by a collateral assignment of lease payments that MGE makes to MGE Power Elm Road for use of the Elm Road Units pursuant to the related long-term leases. MGE Power West Campus's long-term debt includes debt issuance costs of $0.1 million as of December 31, 2019 and 2018. The debt is secured by a collateral assignment of lease payments that MGE makes to MGE Power West Campus for use of the cogeneration facility pursuant to the long-term lease. See Footnote 14 for further information on the long-term debt securities.

[2] MGE Power Elm Road and MGE Power West Campus are not subsidiaries of MGE; however, they have been consolidated in the consolidated financial statements of MGE (see Footnote 3). MGE Power Elm Road and MGE Power West Campus are 100% owned by MGE Power, and MGE Power is 100% owned by MGE Energy. MGE Energy's proportionate share of the equity and net income (through its wholly owned subsidiary MGE Power) of MGE Power Elm Road and MGE Power West Campus is classified within the MGE consolidated financial statements as noncontrolling interest. In 2017, there was a $14.8 million and $6.8 million one-time tax impact as a result of the Tax Act for MGE Power Elm Road and MGE Power West Campus, respectively. The Tax Act reduced the federal tax rate from 35% to 21%.