Joint Plant Ownership (Details) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2021 USD ($) MW	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) MW	Sep. 30, 2020 USD ($)
Badger Hollow I Units [Member]
Jointly Owned Utility Plant Interests [Line Items]
Projected Solar Farm Project Costs	[1]	$ 65.0		$ 65.0
Public Utilities Property Plant And Equipment Construction Work In Progress	[1],[2]	$ 57.8		$ 57.8
Badger Hollow I Units [Member] | Public Service Commission of Wisconsin [Member]
Allowance for Funds Used During Construction
Authorized AFUDC rate - Signficiant projects (100%)				100.00%
Badger Hollow I Units [Member] | MGE [Member]
Jointly Owned Utility Plant Interests [Line Items]
Jointly owned utility plant, ownership interest	[1]	33.00%		33.00%
Jointly owned utility plant, plant capacity (in MW) | MW	[1]	50		50
Badger Hollow II Units [Member]
Jointly Owned Utility Plant Interests [Line Items]
Projected Solar Farm Project Costs	[1]	$ 65.0		$ 65.0
Public Utilities Property Plant And Equipment Construction Work In Progress	[1],[2]	$ 11.5		$ 11.5
Badger Hollow II Units [Member] | Public Service Commission of Wisconsin [Member]
Allowance for Funds Used During Construction
Authorized AFUDC rate - Signficiant projects (100%)				100.00%
Badger Hollow II Units [Member] | MGE [Member]
Jointly Owned Utility Plant Interests [Line Items]
Jointly owned utility plant, ownership interest	[1]	33.00%		33.00%
Jointly owned utility plant, plant capacity (in MW) | MW	[1]	50		50
Badger Hollow I and II [Member]
Allowance for Funds Used During Construction
AFUDC		$ 1.3	$ 0.9	$ 3.6	$ 1.8

[1] The Badger Hollow I and Badger Hollow II solar farms are located in southwestern Wisconsin in Iowa County, near the villages of Montfort and Cobb.

[2] Excluding AFUDC.