Employee Benefit Plans (Tables)

12 Months Ended

Dec. 31, 2016

Compensation and Retirement Disclosure [Abstract]

Schedule of Changes in Projected Benefit Obligations

Following is a reconciliation of the changes in plan benefit obligations and fair value of plan assets, and a statement of the funded status (in thousands):

	Pension Benefits						Other Postretirement Benefits
	December 31,						December 31,
	2016			2015			2016			2015
Change in benefit obligation:
Obligation at beginning of period	$	628,883		$	688,444		$	28,652		$	30,004
Service cost	11,759			12,362			492			526
Interest cost	26,210			26,174			795			786
Plan amendments	—			—			—			1,045
Actuarial loss (gain)	7,006			(47,351		)	(71		)	(616		)
Settlements	—			—			390			390
Benefits paid	(27,826		)	(50,746		)	(4,041		)	(3,483		)
Benefit Obligation at End of Period	$	646,032		$	628,883		$	26,217		$	28,652
Change in Fair Value of Plan Assets:
Fair value of plan assets at beginning of period	$	500,044		$	556,051		$	17,972		$	18,040
Return on plan assets	39,719			(15,461		)	1,277			—
Employer contributions	12,700			10,200			3,397			3,415
Benefits paid	(27,826		)	(50,746		)	(4,041		)	(3,483		)
Fair value of plan assets at end of period	$	524,637		$	500,044		$	18,605		$	17,972
Funded Status	$	(121,395	)	$	(128,839	)	$	(7,612	)	$	(10,680	)
Amounts Recognized in the Balance Sheet Consist of:
Current liability	—			—			(1,789		)	(2,584		)
Noncurrent liability	(121,395		)	(128,839		)	(5,823		)	(8,096		)
Net amount recognized	$	(121,395	)	$	(128,839	)	$	(7,612	)	$	(10,680	)
Amounts Recognized in Regulatory Assets Consist of:
Prior service (cost) credit	(9		)	(255		)	11,988			14,021
Net actuarial loss	(127,953		)	(142,305		)	(4,739		)	(5,219		)
Amounts recognized in AOCL consist of:
Prior service cost	—			—			(849		)	(1,000		)
Net actuarial gain	—			—			38			(102		)
Total	$	(127,962	)	$	(142,560	)	$	6,438		$	7,700

Schedule of Benefit Obligations in Excess of Fair Value of Plan Assets

The total projected benefit obligation and fair value of plan assets for the pension plans with accumulated benefit obligations in excess of plan assets were as follows (in millions):

	Pension Benefits
	December 31,
	2016		2015
Projected benefit obligation	$	646.0	$	628.9
Accumulated benefit obligation	643.6		626.0
Fair value of plan assets	524.6		500.0

Schedule of Defined Benefit Plans Disclosures

The components of the net costs (credits) for our pension and other postretirement plans are as follows (in thousands):

	Pension Benefits									Other Postretirement Benefits
	December 31,									December 31,
	2016			2015			2014			2016			2015			2014
Components of Net Periodic Benefit Cost
Service cost	$	11,759		$	12,362		$	10,830		$	492		$	526		$	465
Interest cost	26,210			26,174			26,147			795			786			859
Expected return on plan assets	(28,248		)	(31,561		)	(29,506		)	(1,042		)	(969		)	(981		)
Amortization of prior service cost (credit)	246			246			246			(1,882		)	(1,882		)	(1,998		)
Recognized actuarial loss	9,888			10,634			2,118			315			385			348
Settlement loss recognized	—			—			—			390			390			690
Net Periodic Benefit Cost (Credit)	$	19,855		$	17,855		$	9,835		$	(932	)	$	(764	)	$	(617	)

Schedule of Estimated Amortization of Regulatory Assets Into Net Periodic Benefit Costs

We estimate amortizations from regulatory assets into net periodic benefit cost during 2017 will be as follows (in thousands):

	Pension Benefits			Other Postretirement Benefits
Prior service credit (cost)	$	(9	)	$	1,882
Accumulated loss	(7,901		)	(313		)

Schedule of Assumptions Used

The weighted-average assumptions used in calculating the preceding information are as follows:

	Pension Benefits						Other Postretirement Benefits
	December 31,						December 31,
	2016		2015		2014		2016		2015		2014
Discount rate	3.95-4.10	%	4.15-4.30	%	3.75-3.90	%	3.40-3.55	%	3.60-3.75	%	3.20-3.40	%
Expected rate of return on assets	5.80		5.80		5.80		5.80		5.80		5.80
Long-term rate of increase in compensation levels (nonunion)	3.28		3.58		3.58		3.28		3.58		3.58
Long-term rate of increase in compensation levels (union)	3.20		3.50		3.50		3.20		3.50		3.50

Schedule of Pension And Postretirement Benefits Investment Strategy

Based on this, the target asset allocation established, within an allowable range of plus or minus 5%, is as follows:

	Pension Benefits				Other Benefits
	December 31,				December 31,
	2016		2015		2016		2015
Domestic debt securities	55.0	%	55.0	%	40.0	%	40.0	%
International debt securities	5.0		5.0		—		—
Domestic equity securities	34.0		34.0		50.0		50.0
International equity securities	6.0		6.0		10.0		10.0

Schedule of Allocation of Plan Assets

The actual allocation by plan is as follows:

	NorthWestern Energy Pension				NorthWestern Corporation Pension				NorthWestern Energy Health and Welfare
	December 31,				December 31,				December 31,
	2016		2015		2016		2015		2016		2015
Cash and cash equivalents	—	%	0.4	%	0.1	%	—	%	1.0	%	0.1	%
Domestic debt securities	53.4		54.9		64.4		65.8		37.0		37.0
International debt securities	4.6		4.7		4.4		4.5		—		—
Domestic equity securities	36.0		33.9		26.0		24.9		52.6		54.2
International equity securities	6.0		6.1		5.1		4.8		9.4		8.7
	100.0	%	100.0	%	100.0	%	100.0	%	100.0	%	100.0	%

Schedule of Pension Contributions

Annual contributions to each of the pension plans are as follows (in thousands):

	2016		2015		2014
NorthWestern Energy Pension Plan (MT)	$	11,500	$	9,000	$	9,000
NorthWestern Corporation Pension Plan (SD and NE)	1,200		1,200		1,200
	$	12,700	$	10,200	$	10,200

Schedule of Expected Benefit Payments

We estimate the plans will make future benefit payments to participants as follows (in thousands):

	Pension Benefits		Other Postretirement Benefits
2017	$	30,637	$	3,513
2018	32,346		3,464
2019	33,574		3,218
2020	34,847		2,844
2021	35,906		2,634
2022-2026	198,236		9,195