Variable Interest Entities (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) MW		Dec. 31, 2016 USD ($) MW
Significant Balance Sheet Accounts Related to Interests in Variable Interest Entities
Property, plant, and equipment, net	$ 1,283,313		$ 1,245,269
Construction work in progress	58,044		36,790
Accrued interest and taxes	5,602		5,495
Deferred income taxes	225,130		$ 383,813
Long-term debt, gross, current and noncurrent	$ 426,883
Other Variable Interest Entities
Other variable interest entities, purchase power agreement, plant capacity (in MW) | MW	61		51
Tax Cuts and Jobs Act [Member]
Effects Of The Tax Cuts And Jobs Act [Abstract]
Deferred Income Tax Expense Tax Cuts And Jobs Act	$ (21,700)
MGE Power Elm Road [Member]
Significant Balance Sheet Accounts Related to Interests in Variable Interest Entities
Property, plant, and equipment, net	176,002		$ 175,502
Construction work in progress	1,225		3,241
Deferred income taxes	29,256	[1]	42,525
Long-term debt	59,416		62,021
Noncontrolling interest	97,635	[1]	80,362
Long-term debt, gross, current and noncurrent	60,000
Total carrying costs	62,500
Total capitalized interest	17,000
Total capitalized cost of equity	$ 45,500
Carrying cost rate recovery period	6 years
MGE Power Elm Road [Member] | Tax Cuts and Jobs Act [Member]
Effects Of The Tax Cuts And Jobs Act [Abstract]
Deferred Income Tax Expense Tax Cuts And Jobs Act	$ (14,800)
MGE Power Elm Road [Member] | Minimum [Member]
Significant Balance Sheet Accounts Related to Interests in Variable Interest Entities
Debt service coverage ratio	1.25
MGE Power West Campus [Member]
Significant Balance Sheet Accounts Related to Interests in Variable Interest Entities
Property, plant, and equipment, net	$ 80,282		81,588
Affiliate receivables, current and noncurrent	4,236		4,769
Deferred income taxes	13,795	[2]	19,912
Long-term debt	43,262		44,932
Noncontrolling interest	43,267	[2]	$ 35,303
Long-term debt, gross, current and noncurrent	43,400
MGE Power West Campus [Member] | Tax Cuts and Jobs Act [Member]
Effects Of The Tax Cuts And Jobs Act [Abstract]
Deferred Income Tax Expense Tax Cuts And Jobs Act	$ (6,800)
MGE Power West Campus [Member] | Minimum [Member]
Significant Balance Sheet Accounts Related to Interests in Variable Interest Entities
Debt service coverage ratio	1.25
MGE Power West Campus [Member] | Maximum [Member]
Significant Balance Sheet Accounts Related to Interests in Variable Interest Entities
Debt to total capitalization ratio	0.65

[1] In 2017, there was a $ 14.8 million one-time tax impact as a result of the Tax Act for MGE Power Elm Road. See Footnote 12 for further information.

[2] In 2017, there was a $ 6.8 million one-time tax impact as a result of the Tax Act for MGE Power West Campus. See Footnote 12 for further information.