Employee Benefit Plans Benefit Obligation And Funded Status (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018
Pension Plan [Member]
Change in Benefit Obligation:
Obligation at beginning of period		$ 649,626	$ 696,796
Service cost		9,637	11,776
Interest cost		26,488	24,420
Actuarial loss (gain)		83,364	(53,496)
Settlements		(4,065)	0
Benefits Paid		(29,486)	(29,870)
Benefit Obligation at End of Period		735,564	649,626
Amounts Recognized in the Balance Sheet Consist of:
Noncurrent asset		4,333	2,672
Total Assets		4,333	2,672
Current liability		(11,401)	0
Noncurrent liability		(119,496)	(126,988)
Total Liabilities		(130,897)	(126,988)
Net amount recognized		(126,564)	(124,316)
Amounts recognized in AOCL consist of:
Prior service cost		0	0
Net actuarial gain		0	0
Total		(111,449)	(116,425)
Plans with Benefit Obligations in Excess of Plan Assets [Abstract]
Projected benefit obligation	[1]	675,500	592,500
Accumulated benefit obligation	[1]	675,500	592,500
Fair value of plan assets	[1]	545,800	466,700
Pension Plan [Member] | Pension Costs [Member]
Amounts Recognized in Regulatory Assets Consist of:
Prior service credit		0	0
Net actuarial (loss) gain		(111,449)	(116,425)
Pension Plan [Member] | Changes Measurement [Member]
Change in Fair Value of Plan Assets:
Fair value of plan assets at beginning of period		525,310	586,508
Return on plan assets		107,041	(40,528)
Employer contributions		10,200	9,200
Settlements		(4,065)	0
Benefits Paid		(29,486)	(29,870)
Fair value of plan assets at end of period		609,000	525,310
Funded Status		(126,564)	(124,316)
Other Postretirement Benefits Plan [Member]
Change in Benefit Obligation:
Obligation at beginning of period		20,611	22,921
Service cost		331	398
Interest cost		609	578
Actuarial loss (gain)		997	(1,903)
Settlements		390	390
Benefits Paid		(2,666)	(1,773)
Benefit Obligation at End of Period		20,272	20,611
Amounts Recognized in the Balance Sheet Consist of:
Noncurrent asset		7,783	4,565
Total Assets		7,783	4,565
Current liability		(2,113)	(2,271)
Noncurrent liability		(4,463)	(4,235)
Total Liabilities		(6,576)	(6,506)
Net amount recognized		1,207	(1,941)
Amounts recognized in AOCL consist of:
Prior service cost		(397)	(548)
Net actuarial gain		934	(1,260)
Total		6,686	6,724
Other Postretirement Benefits Plan [Member] | Pension Costs [Member]
Amounts Recognized in Regulatory Assets Consist of:
Prior service credit		5,890	7,922
Net actuarial (loss) gain		259	(1,910)
Other Postretirement Benefits Plan [Member] | Changes Measurement [Member]
Change in Fair Value of Plan Assets:
Fair value of plan assets at beginning of period		18,670	20,380
Return on plan assets		3,805	(866)
Employer contributions		1,670	929
Settlements		0	0
Benefits Paid		(2,666)	(1,773)
Fair value of plan assets at end of period		21,479	18,670
Funded Status		$ 1,207	$ (1,941)

[1] As of December 31, 2019, the fair value of the NorthWestern Corporation pension plan assets exceed the total projected and accumulated benefit obligation and are therefore excluded from this table.