Noncontrolling Interest (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017		Dec. 31, 2016	Dec. 31, 2015
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 [Member]
Noncontrolling Interest [Line Items]
Noncontrolling interest on balance sheet		140,902		$ 115,665
Net income attributable to noncontrolling interest, net of tax		43,237		23,358	$ 26,097
MGE [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		(21,700)
MGE Transco [Member]
Noncontrolling Interest [Line Items]
Net income attributable to noncontrolling interest, net of tax	[1]	$ 0		1,410	2,172
MGE Power [Member]
Noncontrolling Interest [Line Items]
Ownership percentage by parent		100.00%
MGE Power West Campus [Member]
Noncontrolling Interest [Line Items]
Ownership percentage by parent		100.00%
Noncontrolling interest on balance sheet		$ 43,267	[2]	35,303
Net income attributable to noncontrolling interest, net of tax	[3]	13,963		7,200	7,348
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 Elm Road [Member]
Noncontrolling Interest [Line Items]
Ownership percentage by parent		100.00%
Noncontrolling interest on balance sheet		$ 97,635	[4]	80,362
Net income attributable to noncontrolling interest, net of tax	[3]	29,274		$ 14,748	$ 16,577
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)

[1] At December 31, 2017 , MGE Energy is the owner of MGE Transco. In July 2016, MGE's ownership interest in MGE Transco declined below a majority as a result of continued funding of ATC capital contributions by MGE Energy. As a result of the change in majority owner ship in MGE Transco , MGE deconsolidated MGE Energy's pro portionate share of the equity in MGE Transco. The change in consolidation was applied prospectively by reducing its investment and noncontrolling interest on MGE's consolidated financial statements. The change had no effect on MGE Energy's consolidated fi nancial statements; however, MGE Energy's proportionate share of the equity and net income of MGE Transco classified as noncontrolling interest was deconsolidated from MGE ' s financial statements. No gain or loss was recognized in July 2016 due to MGE ceasi ng to have a controlling financial interest .

[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.

[3] MGE Power Elm Road and MGE Power West Campus are not su bsidiaries of MGE; however, they have been consolidated in the consolidated financial statements of MGE (see Footnote 2). MGE Power Elm Road and MGE Power West Campus are 100 % owned by MGE Power, and MGE Power is 100 % o wned 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 Ac t for MGE Power Elm Road and MGE Power West Campus, respectively . The Tax Act reduced the federal tax rate from 35% to 21% . S ee Footnote 12 for further information .

[4] 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.