Employee Benefit Plans (Tables)

12 Months Ended

Dec. 31, 2014

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,
	2014			2013			2014			2013
Change in Benefit Obligation:
Obligation at beginning of period	$	567,866		$	609,643		$	30,084		$	34,040
Service cost	10,830			13,465			465			541
Interest cost	26,147			22,719			859			877
Actuarial loss (gain)	107,023			(54,671		)	958			(3,156		)
Settlements	—			—			690			—
Benefits paid	(23,422		)	(23,290		)	(3,052		)	(2,218		)
Benefit obligation at end of period	$	688,444		$	567,866		$	30,004		$	30,084
Change in Fair Value of Plan Assets:
Fair value of plan assets at beginning of period	$	516,352		$	472,936		$	18,183		$	15,893
Return on plan assets	52,921			55,006			1,391			2,662
Employer contributions	10,200			11,700			1,518			1,846
Benefits paid	(23,422		)	(23,290		)	(3,052		)	(2,218		)
Fair value of plan assets at end of period	$	556,051		$	516,352		$	18,040		$	18,183
Funded Status	$	(132,393	)	$	(51,514	)	$	(11,964	)	$	(11,901	)
Amounts recognized in the balance sheet consist of:
Current liability	—			—			(1,169		)	(1,178		)
Noncurrent liability	(132,393		)	(51,514		)	(10,795		)	(10,723		)
Net amount recognized	$	(132,393	)	$	(51,514	)	$	(11,964	)	$	(11,901	)
Amounts recognized in regulatory assets consist of:
Prior service (cost) credit	(502		)	(748		)	17,098			19,247
Net actuarial loss	(153,268		)	(71,777		)	(4,945		)	(4,807		)
Amounts recognized in AOCI consist of:
Prior service cost	—			—			(1,151		)	(1,302		)
Net actuarial gain	—			—			(409		)	(971		)
Total	$	(153,770	)	$	(72,525	)	$	10,593		$	12,167

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,
	2014		2013
Projected benefit obligation	$	688.4	$	567.9
Accumulated benefit obligation	685.0		565.0
Fair value of plan assets	556.1		516.4

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,
	2014			2013			2012			2014			2013			2012
Components of Net Periodic Benefit Cost
Service cost	$	10,830		$	13,465		$	11,488		$	465		$	541		$	541
Interest cost	26,147			22,719			23,823			859			877			1,167
Expected return on plan assets	(29,506		)	(32,491		)	(29,996		)	(981		)	(1,019		)	(1,021		)
Amortization of prior service cost (credit)	246			246			246			(1,998		)	(1,998		)	(1,998		)
Recognized actuarial loss	2,118			11,648			8,646			348			1,271			790
Settlement loss recognized	—			—			—			690			—			—
Net Periodic Benefit Cost (Credit)	$	9,835		$	15,587		$	14,207		$	(617	)	$	(328	)	$	(521	)

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

We estimate amortizations from regulatory assets into net periodic benefit cost during 2015 will be as follows (in thousands):

	Pension Benefits			Other Postretirement Benefits
Prior service credit (cost)	$	(246	)	$	1,998
Accumulated loss	(10,470		)	(325		)

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,
	2014		2013		2012		2014		2013		2012
Discount rate	3.75-3.90	%	4.55-4.75	%	3.55-3.80	%	3.20-3.40	%	3.75-4.20	%	2.25-3.20	%
Expected rate of return on assets	5.80		7.00		7.00		5.80		7.00		7.00
Long-term rate of increase in compensation levels (nonunion)	3.58		3.58		3.58		3.58		3.58		3.58
Long-term rate of increase in compensation levels (union)	3.50		3.50		3.50		3.50		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,
	2014		2013		2014		2013
Domestic debt securities	55.0	%	60.0	%	40.0	%	40.0	%
International debt securities	5.0		5.0		—		—
Domestic equity securities	34.0		30.0		50.0		50.0
International equity securities	6.0		5.0		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,
	2014		2013		2014		2013		2014		2013
Cash and cash equivalents	—	%	—	%	0.1	%	0.1	%	0.2	%	1.8	%
Domestic debt securities	56.0		58.6		65.6		64.7		37.2		38.6
International debt securities	4.4		4.9		4.5		4.9		—		0.3
Domestic equity securities	34.1		31.4		25.1		25.3		53.9		50.1
International equity securities	5.5		5.1		4.7		5.0		8.7		9.2
	100.0	%	100.0	%	100.0	%	100.0	%	100.0	%	100.0	%

Schedule of Fair Value of Plan Assets by Category

The fair value of our plan assets at December 31, 2014, by asset category are as follows (in thousands):

Asset Category	Total		Quoted Market Prices in Active Markets for Identical Assets Level 1		Significant Observable Inputs Level 2		Significant Unobservable Inputs Level 3
Pension Plan Assets
Cash and cash equivalents	$	126	$	—	$	126	$	—
Equity securities: (1)
US small/mid cap growth	16,605		—		16,605		—
US small/mid cap value	15,264		—		15,264		—
US large cap growth	48,560		—		48,560		—
US large cap value	48,785		—		48,785		—
US large cap passive	54,775		—		54,775		—
Non-US core	22,634		—		22,634		—
Emerging markets	7,650		—		7,650		—
Fixed income securities:(2)
US core	23,177		—		23,177		—
US passive	12,269		—		12,269		—
Long duration	41,451		—		41,451		—
Long duration investment grade	52,767		—		52,767		—
Long duration passive	41,475		—		41,475		—
Opportunistic	5,692		—		5,692		—
Non-US passive	24,504		—		24,504		—
Active long corporate	133,160		—		133,160		—
Participating group annuity contract	7,157		—		7,157		—
	$	556,051	$	—	$	556,051	$	—
Other Postretirement Benefit Plan Assets
Cash and cash equivalents	$	44	$	—	$	44	$	—
Equity securities: (1)					—
US small/mid cap growth	752		—		752		—
US small/mid cap value	721		—		721		—
S&P 500 index	8,234		—		8,234		—
US large cap growth	6		—		6		—
US large cap value	6		—		6		—
US large cap passive	7		—		7		—
Non-US core	1,495		—		1,495		—
Emerging markets	72		—		72		—
Fixed income securities: (2)
Passive bond market	1,992		—		1,992		—
US core	4,435		—		4,435		—
US passive	1		—		1		—
Long duration	5		—		5		—
Long duration investment grade	6		—		6		—
Long duration passive	5		—		5		—
Opportunistic	240		—		240		—
Non-US passive	3		—		3		—
Active long corporate	16		—		16		—
	$	18,040	$	—	$	18,040	$	—

The fair value of our plan assets at December 31, 2013, by asset category are as follows (in thousands):

Asset Category	Total		Quoted Market Prices in Active Markets for Identical Assets Level 1		Significant Observable Inputs Level 2		Significant Unobservable Inputs Level 3
Pension Plan Assets
Cash and cash equivalents	$	168	$	—	$	168	$	—
Equity securities: (1)
US small/mid cap growth	13,764		—		13,764		—
US small/mid cap value	13,664		—		13,664		—
US large cap growth	42,094		—		42,094		—
US large cap value	42,102		—		42,102		—
US large cap passive	47,227		—		47,227		—
Non-US core	20,015		—		20,015		—
Emerging markets	6,250		—		6,250		—
Fixed income securities:(2)
US core	82,639		—		82,639		—
US passive	44,762		—		44,762		—
Long duration	24,401		—		24,401		—
Long duration investment grade	32,700		—		32,700		—
Long duration passive	24,122		—		24,122		—
Opportunistic	5,876		—		5,876		—
Non-US passive	25,150		—		25,150		—
Active long corporate	83,147		—		83,147		—
Participating group annuity contract	8,271		—		8,271		—
	$	516,352	$	—	$	516,352	$	—
Other Postretirement Benefit Plan Assets
Cash and cash equivalents	$	318	—		$	318	—
Equity securities: (1)
US small/mid cap growth	751		—		751		—
US small/mid cap value	736		—		736		—
S&P 500 index	7,321		—		7,321		—
US large cap growth	98		—		98		—
US large cap value	98		—		98		—
US large cap passive	110		—		110		—
Non-US core	1,595		—		1,595		—
Emerging markets	85		—		85		—
Fixed income securities: (2)
Passive bond market	1,880		—		1,880		—
US core	4,390		—		4,390		—
US passive	107		—		107		—
Long duration	55		—		55		—
Long duration investment grade	79		—		79		—
Long duration passive	55		—		55		—
Opportunistic	261		—		261		—
Non-US passive	57		—		57		—
Active long corporate	187		—		187		—
	$	18,183	$	—	$	18,183	$	—

_________________

(1) This category consists of active and passive managed equity funds, which are invested in multiple strategies to diversify risks and reduce volatility.

(2) This category consists of investment grade bonds of issuers from diverse industries, debt securities issued by international, national, state and local governments, and asset-backed securities. This includes both active and passive managed funds.

Schedule of Pension Contributions

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

	2014		2013		2012
NorthWestern Energy Pension Plan (MT)	$	9,000	$	10,500	$	10,500
NorthWestern Pension Plan (SD)	1,200		1,200		1,200
	$	10,200	$	11,700	$	11,700

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
2015	$	27,652	$	3,516
2016	29,905		3,516
2017	31,172		3,387
2018	33,142		3,282
2019	34,660		3,026
2020-2024	194,065		11,923