Joint Plant Ownership (Details) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2020 USD ($) MW	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) MW	Sep. 30, 2019 USD ($)
Allowance for Funds Used During Construction
AFUDC		$ 1.9	$ 0.7	$ 4.5	$ 0.8
Two Creeks 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]	$ 63.1		$ 63.1
Two Creeks Units [Member] | Public Service Commission of Wisconsin [Member]
Allowance for Funds Used During Construction
Authorized AFUDC rate - Signficiant projects (100%)		100.00%		100.00%
Two Creeks 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 Units [Member]
Jointly Owned Utility Plant Interests [Line Items]
Projected Solar Farm Project Costs	[3]	$ 65.0		$ 65.0
Public Utilities Property Plant And Equipment Construction Work In Progress	[2],[3]	$ 50.5		$ 50.5
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%		100.00%
Badger Hollow I Units [Member] | MGE [Member]
Jointly Owned Utility Plant Interests [Line Items]
Jointly owned utility plant, ownership interest	[3]	33.00%		33.00%
Jointly owned utility plant, plant capacity (in MW) | MW	[3]	50		50
Badger Hollow II Units [Member]
Jointly Owned Utility Plant Interests [Line Items]
Projected Solar Farm Project Costs	[3]	$ 65.0		$ 65.0
Public Utilities Property Plant And Equipment Construction Work In Progress	[2],[3]	$ 5.0		$ 5.0
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%		100.00%
Badger Hollow II Units [Member] | MGE [Member]
Jointly Owned Utility Plant Interests [Line Items]
Jointly owned utility plant, ownership interest	[3]	33.00%		33.00%
Jointly owned utility plant, plant capacity (in MW) | MW	[3]	50		50

[1] The Two Creeks solar generation array is located in the Town of Two Creeks and the City of Two Rivers in Manitowoc and Kewaunee Counties, Wisconsin.

[2] Excluding AFUDC.

[3] The Badger Hollow I and Badger Hollow II solar farm is located in southwestern Wisconsin in Iowa County, near the villages of Montfort and Cobb.   (d)Estimated date of commercial operation.