Pension Plans and Other Postretirement Benefits (Details-1) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2016		Dec. 31, 2015	Dec. 31, 2014
Defined contribution plans [Abstract]
Defined contribution costs		$ 3,100		$ 2,800	$ 2,500
Change in plan assets [Roll Forward]
Fair value of plan assets at beginning of year		330,886
Fair value of plan assets at end of year		355,110		330,886
Amounts recognized in the consolidated balance sheets to reflect funded status of plans [Abstract]
Long-term asset		2,020		0
Long-term liability		(74,347)		(75,680)
Pension Benefits [Member]
Change in benefit obligations [Roll Forward]
Net benefit obligation at beginning of year		332,565		340,233
Service cost		5,365		7,263	6,179
Interest cost		12,393		13,766	13,574
Plan participants' contributions		0		0
Actuarial loss (gain)		11,412	[1]	(17,576)
Gross benefits paid		(12,179)		(11,121)
Less: federal subsidy on benefits paid		0		0
Benefit obligation at end of year		349,556		332,565	340,233
Change in plan assets [Roll Forward]
Fair value of plan assets at beginning of year		290,716		288,548
Actual return on plan assets		24,181		4,153
Employer contributions		9,215		9,136
Plan participants' contributions		0		0
Gross benefits paid		(12,179)		(11,121)
Fair value of plan assets at end of year		311,933		290,716	288,548
Funded Status at December 31		(37,623)		(41,849)
Accumulated benefit obligation		319,900		302,500
Amounts recognized in the consolidated balance sheets to reflect funded status of plans [Abstract]
Long-term asset		2,020		0
Current liability		(972)		(966)
Long-term liability		(38,671)		(40,883)
Net liability		(37,623)		(41,849)
Amounts recognized in the consolidated balance sheets as regulatory asset [Abstract]
Net actuarial loss		84,656		80,660
Prior service (credit) cost		(446)		(436)
Transition obligation		0		0
Total		84,210		80,224
Projected benefit obligation in excess of plan assets [Abstract]
Projected benefit obligation with projected benefit obligation in excess of plan assets, end of year		227,739		332,565
Fair value of plan assets with projected benefit obligation in excess of plan assets, end of year		188,096		290,716
Accumulated benefit obligation in excess of plan assets [Abstract]
Accumulated benefit obligation with accumulated benefit obligation in excess of plan assets, end of year		26,927		24,606
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		74,935		78,478
Service cost		1,271		1,559	1,339
Interest cost		2,681		3,075	3,166
Plan participants' contributions		767		741
Actuarial loss (gain)		2,638	[1]	(5,828)
Gross benefits paid		(3,637)		(3,280)
Less: federal subsidy on benefits paid	[2]	187		190
Benefit obligation at end of year		78,842		74,935	78,478
Change in plan assets [Roll Forward]
Fair value of plan assets at beginning of year		40,170		38,952
Actual return on plan assets		3,236		603
Employer contributions		2,641		3,154
Plan participants' contributions		767		741
Gross benefits paid		(3,637)		(3,280)
Fair value of plan assets at end of year		43,177		40,170	$ 38,952
Funded Status at December 31		(35,665)		(34,765)
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		(52)
Long-term liability		(35,665)		(34,713)
Net liability		(35,665)		(34,765)
Amounts recognized in the consolidated balance sheets as regulatory asset [Abstract]
Net actuarial loss		14,728		13,086
Prior service (credit) cost		(12,489)		(15,158)
Transition obligation		26		29
Total		$ 2,265		$ (2,043)

[1] In 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, 2016 and 2015 , the subsidy due to MGE was $ 0.2 million.