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Employee Benefit Plans (Tables)
 12 Months Ended
Dec. 31, 2024
Retirement Benefits [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 BenefitsOther Postretirement Benefits
 December 31,December 31,
 2024202320242023
Change in benefit obligation:    
Obligation at beginning of period$473,988 $521,798 $13,708 $15,407 
Service cost5,592 5,646 308 333 
Interest cost22,944 25,852 557 674 
Actuarial (gain) loss(28,499)3,127 (2,514)(1,240)
Settlements(1)
(848)(51,942)— — 
Benefits paid(25,230)(30,493)(1,333)(1,466)
Benefit Obligation at End of Period$447,947 $473,988 $10,726 $13,708 
Change in fair value of plan assets:    
Fair value of plan assets at beginning of period$402,671 $441,539 $22,309 $20,055 
Return on plan assets9,411 34,367 3,177 3,334 
Employer contributions9,322 9,200 619 386 
Settlements(1)
(848)(51,942)— — 
Benefits paid(25,230)(30,493)(1,333)(1,466)
Fair value of plan assets at end of period$395,326 $402,671 $24,772 $22,309 
Funded Status$(52,621)$(71,317)$14,046 $8,601 
Amounts Recognized in the Balance Sheet Consist of:    
Noncurrent asset9,467 7,875 16,943 12,378 
Total Assets9,467 7,875 16,943 12,378 
Current liability(10,000)(11,200)(1,310)(1,355)
Noncurrent liability(52,088)(67,992)(1,587)(2,422)
Total Liabilities(62,088)(79,192)(2,897)(3,777)
Net amount recognized$(52,621)$(71,317)$14,046 $8,601 
Amounts Recognized in Regulatory Assets Consist of:    
Prior service credit— — — — 
Net actuarial (loss) gain(31,835)(44,453)3,716 15 
Amounts recognized in AOCL consist of:
    
Prior service cost— — — — 
Net actuarial gain— — 1,228 590 
Total$(31,835)$(44,453)$4,944 $605 
(1) In October 2023, we entered into a group annuity contract from an insurance company to provide for the payment of pension benefits to select NorthWestern Energy MT Pension Plan participants. We purchased the contract with $51.9 million of plan assets in 2023. A trailing premium of $0.8 million related to final data reconciliation was paid from plan assets in 2024, reflecting a final, annuitized participant count of 276. The insurance company took over the payments of these benefits starting January 1, 2024. This transaction settled $52.7 million of our NorthWestern Energy MT Pension Plan obligation. As a result of this transaction, during the twelve months ended December 31, 2023, we recorded a non-cash, non-operating settlement charge of $4.4 million. This charge is recorded within other income, net on the Consolidated Statements of Income. As discussed within Note 4 – Regulatory Assets and Liabilities, the MPSC allows recovery of pension costs on a cash funding basis. As such, this charge was deferred as a regulatory asset on the Consolidated Balance Sheets, with a corresponding decrease to operating and maintenance expense on the Consolidated Statements of Income.
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 NorthWestern Energy MT Pension Plan with accumulated benefit obligations in excess of plan assets were as follows (in millions):
 NorthWestern Energy MT Pension Plan
 December 31,
20242023
Projected benefit obligation$404.8 $427.3 
Accumulated benefit obligation404.8 427.3 
Fair value of plan assets342.7 348.1 

    As of December 31, 2024, the fair value of the NorthWestern Energy SD/NE Pension Plan assets exceeds the total projected and accumulated benefit obligation and are therefore excluded from this table.
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 BenefitsOther Postretirement Benefits
 December 31,December 31,
 202420232022202420232022
Components of net periodic benefit cost      
Service cost$5,592 $5,646 $10,223 $308 $333 $351 
Interest cost22,944 25,852 18,787 557 674 359 
Expected return on plan assets(25,325)(25,932)(24,173)(1,280)(1,096)(1,047)
Amortization of prior service cost (credit)— — — — 116 (1,891)
Recognized actuarial loss (gain)33 228 383 (73)(672)(897)
Settlement loss recognized(1)
— 4,395 — — — — 
Net Periodic Benefit Cost (Credit)$3,244 $10,189 $5,220 $(488)$(645)$(3,125)
Regulatory deferral of net periodic benefit cost(2)
4,850 (1,814)2,307 — — — 
Previously deferred costs recognized(2)
75 210 — 181 550 292 
Net Periodic Benefit Cost (Credit) Recognized$8,169 $8,585 $7,527 $(307)$(95)$(2,833)
(1) Settlement loss is related to partial annuitization of NorthWestern Energy MT Pension Plan effective October 24, 2023.
(2) Net periodic benefit costs for pension and postretirement benefit plans are recognized for financial reporting based on the authorization of each regulatory jurisdiction in which we operate. A portion of these costs are recorded in regulatory assets and recognized in the Consolidated Statements of Income as those costs are recovered through customer rates.
For the years ended December 31, 2024, 2023, and 2022, Service costs were recorded in Operations and maintenance expense while non-service costs were recorded in Other income, net on the Consolidated Statements of Income.
Schedule of Assumptions Used
The weighted-average assumptions used in calculating the preceding information are as follows:
 Pension BenefitsOther Postretirement Benefits
 December 31,December 31,
 202420232022202420232022
Discount rate5.50-5.60%4.95-5.005.20 %5.30-5.45%4.85-4.90%5.15-5.20%
Expected rate of return on assets5.15-6.654.83-6.442.66-4.265.84 5.62 4.23 
Long-term rate of increase in compensation levels (non-union)4.00 4.00 4.00 4.00 4.00 4.00 
Long-term rate of increase in compensation levels (union)4.00 4.00 4.00 4.00 4.00 4.00 
Interest crediting rate3.3-6.03.30-6.003.30-6.00N/AN/AN/A
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 percent, is as follows:
 NorthWestern Energy MT PensionNorthWestern Energy SD/NE PensionNorthWestern Energy
Health and Welfare
 December 31,December 31,December 31,
 202420232024202320242023
Fixed income securities45.0 %45.0 %90.0 %90.0 %40.0 %40.0 %
Non-U.S. fixed income securities— — — — — — 
Opportunistic fixed income11.0 11.0 3.0 3.0 — — 
Global equities38.5 38.5 7.0 7.0 60.0 60.0 
Private real estate5.5 5.5 — — — — 
Schedule of Allocation of Plan Assets
The actual allocation by plan is as follows:
 NorthWestern Energy MT PensionNorthWestern Energy SD/NE PensionNorthWestern Energy
Health and Welfare
 December 31,December 31,December 31,
202420232024202320242023
Cash and cash equivalents— %— %0.8 %1.5 %0.3 %0.2 %
Fixed income securities(1)
43.7 45.3 89.4 88.7 32.2 35.1 
Non-U.S. fixed income securities— — — — — — 
Opportunistic fixed income11.1 10.6 2.9 2.9 — — 
Global equities(1)
39.0 37.6 6.9 6.9 67.5 64.7 
Private real estate6.2 6.5 — — — — 
 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):
 202420232022
NorthWestern Energy MT Pension Plan$8,122 $8,000 $7,000 
NorthWestern Energy SD/NE Pension Plan1,200 1,200 1,200 
 $9,322 $9,200 $8,200 
Schedule of Expected Benefit Payments
We estimate the plans will make future benefit payments to participants as follows (in thousands):
 Pension BenefitsOther Postretirement Benefits
202528,549 1,919 
202629,467 1,216 
202730,393 1,064 
202831,155 1,015 
202932,218 935 
2030-2034166,566 4,329