Pension Plans and Other Postretirement Benefits (Details-1) - USD ($)		12 Months Ended
		Dec. 31, 2020		Dec. 31, 2019		Dec. 31, 2018
Defined Benefit Plan, Deferred Pension and Other Postretirement Benefit Plan Costs Recovered [Abstract]
Defined contribution costs		$ 4,700,000		$ 4,400,000		$ 3,900,000
Change in plan assets [Roll Forward]
Fair value of plan assets as of January 1		434,922,000
Fair value of plan assets as of December 31		481,273,000		434,922,000
Amounts recognized in the consolidated balance sheets to reflect funded status of plans [Abstract]
Long-term asset		13,873,000		13,630,000
Long-term liability		(78,168,000)		(68,665,000)
Pension Benefits [Member]
Change in benefit obligations [Roll Forward]
Net benefit obligation as of January 1		410,651,000		360,288,000
Service cost		5,296,000	[1]	4,692,000	[1]	5,723,000
Interest cost		12,210,000	[1]	14,302,000	[1]	12,859,000
Plan participants' contributions		0		0
Actuarial loss	[2]	50,325,000		47,671,000
Gross benefits paid		(17,267,000)		(16,302,000)
Less: federal subsidy on benefits paid	[3]	0		0
Benefit obligation as of December 31		461,215,000		410,651,000		360,288,000
Change in plan assets [Roll Forward]
Fair value of plan assets as of January 1		386,033,000		323,780,000
Actual return on plan assets		58,935,000		76,766,000
Employer contributions		1,837,000		1,789,000
Plan participants' contributions		0		0
Gross benefits paid		(17,267,000)		(16,302,000)
Fair value of plan assets as of December 31		429,538,000		386,033,000		323,780,000
Funded Status as of December 31		(31,677,000)		(24,618,000)
Accumulated benefit obligation		430,300,000		383,000,000.0
Amounts recognized in the consolidated balance sheets to reflect funded status of plans [Abstract]
Long-term asset		13,873,000		13,630,000
Current liability		(2,139,000)		(1,688,000)
Long-term liability		(43,411,000)		(36,560,000)
Net liability		(31,677,000)		(24,618,000)
Amounts recognized in the consolidated balance sheets as regulatory asset [Abstract]
Net actuarial loss		92,553,000		79,290,000
Prior service (credit) cost		(144,000)		(268,000)
Transition obligation		0		0
Total		92,409,000		79,022,000
Projected benefit obligation in excess of plan assets [Abstract]
Projected benefit obligation with projected benefit obligation in excess of plan assets, end of year		45,550,000		38,247,000
Fair value of plan assets with projected benefit obligation in excess of plan assets, end of year		0		0
Accumulated benefit obligation in excess of plan assets [Abstract]
Accumulated benefit obligation with accumulated benefit obligation in excess of plan assets, end of year		43,384,000		35,798,000
Fair value of plan assets with accumulated benefit obligation in excess of plan assets, end of year		0		0
Postretirement Benefits [Member]
Change in benefit obligations [Roll Forward]
Net benefit obligation as of January 1		80,901,000		75,161,000
Service cost		1,264,000	[1]	1,110,000	[1]	1,283,000
Interest cost		2,278,000	[1]	2,893,000	[1]	2,612,000
Plan participants' contributions		1,009,000		969,000
Actuarial loss	[2]	5,907,000		5,045,000
Gross benefits paid		(5,245,000)		(4,497,000)
Less: federal subsidy on benefits paid	[3]	246,000		220,000
Benefit obligation as of December 31		86,360,000		80,901,000		75,161,000
Change in plan assets [Roll Forward]
Fair value of plan assets as of January 1		48,889,000		42,521,000
Actual return on plan assets		6,514,000		9,277,000
Employer contributions		568,000		619,000
Plan participants' contributions		1,009,000		969,000
Gross benefits paid		(5,245,000)		(4,497,000)
Fair value of plan assets as of December 31		51,735,000		48,889,000		$ 42,521,000
Funded Status as of December 31		(34,625,000)		(32,012,000)
Medicare subsidy due to MGE		200,000		200,000
Amounts recognized in the consolidated balance sheets to reflect funded status of plans [Abstract]
Long-term asset		0		0
Current liability		0		0
Long-term liability		(34,625,000)		(32,012,000)
Net liability		(34,625,000)		(32,012,000)
Amounts recognized in the consolidated balance sheets as regulatory asset [Abstract]
Net actuarial loss		10,986,000		8,659,000
Prior service (credit) cost		(1,816,000)		(4,484,000)
Transition obligation		15,000		17,000
Total		9,185,000		4,192,000
Accumulated benefit obligation in excess of plan assets [Abstract]
Accumulated benefit obligation with accumulated benefit obligation in excess of plan assets, end of year		86,360,000		80,901,000
Fair value of plan assets with accumulated benefit obligation in excess of plan assets, end of year		$ 51,735,000		$ 48,889,000

[1] During 2019, MGE deferred approximately $6.2 million of pension and other postretirement costs. During 2020, MGE collected approximately $0.9 million of pension and other postretirement costs, which reduced the amount deferred in 2019. The impact of the deferral has not been reflected in the table above. See Footnote 8 for further information.

[2] In 2020 and 2019, lower discount rates were the primary driver of the actuarial loss.

[3] In 2003, the Medicare Prescription Drug, Improvement and Modernization Act of 2003 was signed into law authorizing Medicare to provide prescription drug benefits to retirees. For the years ended December 31, 2020 and 2019, the subsidy due to MGE was $0.2 million.