Pension Plans and Other Postretirement Benefits (Details-1) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined contribution plans [Abstract]
Defined contribution costs		$ 3,900	$ 3,500	$ 3,100
Change in plan assets [Roll Forward]
Fair value of plan assets at beginning of year		410,121
Fair value of plan assets at end of year		366,301	410,121
Amounts recognized in the consolidated balance sheets to reflect funded status of plans [Abstract]
Long-term asset		0	7,336
Long-term liability		(67,483)	(69,088)
Pension Benefits [Member]
Change in benefit obligations [Roll Forward]
Net benefit obligation at beginning of year		391,269	349,556
Service cost		5,723	5,383	5,365
Interest cost		12,859	12,625	12,393
Plan participants' contributions		0	0
Actuarial (gain) loss	[1]	(34,439)	37,689
Gross benefits paid		(15,124)	(13,984)
Less: federal subsidy on benefits paid		0	0
Benefit obligation at end of year		360,288	391,269	349,556
Change in plan assets [Roll Forward]
Fair value of plan assets at beginning of year		361,651	311,933
Actual return on plan assets		(24,485)	56,987
Employer contributions		1,738	6,715
Plan participants' contributions		0	0
Gross benefits paid		(15,124)	(13,984)
Fair value of plan assets at end of year		323,780	361,651	311,933
Funded Status at December 31		(36,508)	(29,618)
Accumulated benefit obligation		335,800	356,000
Amounts recognized in the consolidated balance sheets to reflect funded status of plans [Abstract]
Long-term asset		0	7,336
Current liability		(1,732)	(1,726)
Long-term liability		(34,776)	(35,228)
Net liability		(36,508)	(29,618)
Amounts recognized in the consolidated balance sheets as regulatory asset [Abstract]
Net actuarial loss		92,978	81,969
Prior service (credit) cost		(385)	(429)
Transition obligation		0	0
Total		92,593	81,540
Projected benefit obligation in excess of plan assets [Abstract]
Projected benefit obligation with projected benefit obligation in excess of plan assets, end of year		360,288	36,954
Fair value of plan assets with projected benefit obligation in excess of plan assets, end of year		323,780	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		31,200	32,813
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 at beginning of year		82,290	78,842
Service cost		1,283	1,231	1,271
Interest cost		2,612	2,666	2,681
Plan participants' contributions		950	894
Actuarial (gain) loss	[1]	(7,555)	2,749
Gross benefits paid		(4,623)	(4,262)
Less: federal subsidy on benefits paid	[2]	204	170
Benefit obligation at end of year		75,161	82,290	78,842
Change in plan assets [Roll Forward]
Fair value of plan assets at beginning of year		48,470	43,177
Actual return on plan assets		(2,780)	7,104
Employer contributions		504	1,557
Plan participants' contributions		950	894
Gross benefits paid		(4,623)	(4,262)
Fair value of plan assets at end of year		42,521	48,470	$ 43,177
Funded Status at December 31		(32,640)	(33,820)
Medicare subsidy due to MGE		200	200
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		(32,640)	(33,820)
Net liability		(32,640)	(33,820)
Amounts recognized in the consolidated balance sheets as regulatory asset [Abstract]
Net actuarial loss		10,569	12,600
Prior service (credit) cost		(7,153)	(9,821)
Transition obligation		21	23
Total		$ 3,437	$ 2,802

[1] In 2018, higher discount rates were the main driver of the actuarial gain. However, in 2017, lower discount rates were the main driver of the actuarial loss.

[2] 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, 2018 and 2017 , the subsidy due to MGE was $ 0.2 million.