Pension Plans and Other Postretirement Benefits (Details-1) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined contribution plans [Abstract]
Defined contribution costs		$ 3,500	$ 3,100	$ 2,800
Change in plan assets [Roll Forward]
Fair value of plan assets at beginning of year		355,110
Fair value of plan assets at end of year		410,121	355,110
Amounts recognized in the consolidated balance sheets to reflect funded status of plans [Abstract]
Long-term asset		7,336	2,020
Long-term liability		(69,088)	(74,347)
Pension Benefits [Member]
Change in benefit obligations [Roll Forward]
Net benefit obligation at beginning of year		349,556	332,565
Service cost		5,383	5,365	7,263
Interest cost		12,625	12,393	13,766
Plan participants' contributions		0	0
Actuarial loss (gain)	[1]	37,689	11,412
Gross benefits paid		(13,984)	(12,179)
Less: federal subsidy on benefits paid		0	0
Benefit obligation at end of year		391,269	349,556	332,565
Change in plan assets [Roll Forward]
Fair value of plan assets at beginning of year		311,933	290,716
Actual return on plan assets		56,987	24,181
Employer contributions		6,715	9,215
Plan participants' contributions		0	0
Gross benefits paid		(13,984)	(12,179)
Fair value of plan assets at end of year		361,651	311,933	290,716
Funded Status at December 31		(29,618)	(37,623)
Accumulated benefit obligation		356,000	319,900
Amounts recognized in the consolidated balance sheets to reflect funded status of plans [Abstract]
Long-term asset		7,336	2,020
Current liability		(1,726)	(972)
Long-term liability		(35,228)	(38,671)
Net liability		(29,618)	(37,623)
Amounts recognized in the consolidated balance sheets as regulatory asset [Abstract]
Net actuarial loss		81,969	84,656
Prior service (credit) cost		(429)	(446)
Transition obligation		0	0
Total		81,540	84,210
Projected benefit obligation in excess of plan assets [Abstract]
Projected benefit obligation with projected benefit obligation in excess of plan assets, end of year		36,954	227,739
Fair value of plan assets with projected benefit obligation in excess of plan assets, end of year		0	188,096
Accumulated benefit obligation in excess of plan assets [Abstract]
Accumulated benefit obligation with accumulated benefit obligation in excess of plan assets, end of year		32,813	26,927
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		78,842	74,935
Service cost		1,231	1,271	1,559
Interest cost		2,666	2,681	3,075
Plan participants' contributions		894	767
Actuarial loss (gain)	[1]	2,749	2,638
Gross benefits paid		(4,262)	(3,637)
Less: federal subsidy on benefits paid	[2]	170	187
Benefit obligation at end of year		82,290	78,842	74,935
Change in plan assets [Roll Forward]
Fair value of plan assets at beginning of year		43,177	40,170
Actual return on plan assets		7,104	3,236
Employer contributions		1,557	2,641
Plan participants' contributions		894	767
Gross benefits paid		(4,262)	(3,637)
Fair value of plan assets at end of year		48,470	43,177	$ 40,170
Funded Status at December 31		(33,820)	(35,665)
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		(33,820)	(35,665)
Net liability		(33,820)	(35,665)
Amounts recognized in the consolidated balance sheets as regulatory asset [Abstract]
Net actuarial loss		12,600	14,728
Prior service (credit) cost		(9,821)	(12,489)
Transition obligation		23	26
Total		$ 2,802	$ 2,265

[1] In 2017 and 2016, 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, 2017 and 2016 , the subsidy due to MGE was $ 0.2 million.