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Employee Benefit Plans (Tables) | 12 Months Ended
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Dec. 31, 2023
Defined Benefit Plan Disclosure [Line Items] |
Schedule of Allocation of Plan Assets | The percentages of total plan asset by investment category for our Pension Plan at December 31 were as follows:
Return-seeking Assets | 2023 | 2022
Equity | 14% | 14%
Real estate | 5% | 7%
Hedge funds | 3% | 3%
Fixed income | 2% | 2%
Total | 24% | 26%
Liability-hedging Assets | 2023 | 2022
Fixed income | 74% | 72%
Cash | 2% | 2%
Total | 76% | 74%
Total Assets | 100% | 100%
Schedule of Employer Contribution to Employee Benefit Plans | Contributions for the years ended December 31 were as follows (in millions):
| 2023 |  | 2022 |
Defined Contribution Plan |  |  |  |
Company retirement contributions | $ | 12.7 |  | $ | 11.9 |
Company matching contributions | $ | 17.1 |  | $ | 16.2 |
Defined Benefit Plans |  |  |  |
Defined Benefit Pension Plan | $ | — |  | $ | — |
Non-Pension Defined Benefit Postretirement Healthcare Plan | $ | 5.4 |  | $ | 6.1 |
Supplemental Non-Qualified Defined Benefit Plans | $ | 3.5 |  | $ | 3.1 |
Schedule of Changes in Projected Benefit Obligations | The following tables provide a reconciliation of the employee benefit plan obligations and fair value of employee benefit plan assets, amounts recognized on our Consolidated Balance Sheets, accumulated benefit obligation and elements of AOCI (in millions):
| Defined Benefit Pension Plan |  | Supplemental Non-qualified Defined Benefit Plans |  | Non-pension Defined Benefit Postretirement Healthcare Plan |  | Defined Benefit Pension Plan |  | Supplemental Non-qualified Defined Benefit Plans |  | Non-pension Defined Benefit Postretirement Healthcare Plan |
| 2023 |  | 2022 |
Accumulated benefit obligation at December 31 | $ | 341.8 |  | $ | 46.7 |  | $ | 51.1 |  | $ | 350.2 |  | $ | 45.2 |  | $ | 49.7 |
Change in benefit obligation: |  |  |  |  |  |  |  |  |  |  |  |
Projected benefit obligation at beginning of year | $ | 358.4 |  | $ | 45.2 |  | $ | 49.7 |  | $ | 478.3 |  | $ | 55.3 |  | $ | 63.5 |
Service cost |  | 2.5 |  |  | 3.1 |  |  | 1.5 |  |  | 3.9 |  |  | (0.8 | ) |  | 1.9 |
Interest cost |  | 17.5 |  |  | 1.5 |  |  | 2.4 |  |  | 10.8 |  |  | 0.8 |  |  | 1.3 |
Actuarial (gain) loss |  | 11.6 |  |  | 0.3 |  |  | 1.7 |  |  | (97.9 | ) |  | (7.0 | ) |  | (12.3 | )
Benefits paid |  | (41.9 | ) |  | (3.4 | ) |  | (5.3 | ) |  | (36.7 | ) |  | (3.1 | ) |  | (6.1 | )
Plan participants’ contributions |  | — |  |  | — |  |  | 1.1 |  |  | — |  |  | — |  |  | 1.4 |
Projected benefit obligation at end of year |  | 348.1 |  |  | 46.7 |  |  | 51.1 |  |  | 358.4 |  |  | 45.2 |  |  | 49.7 |
Change in fair value of plan assets: |  |  |  |  |  |  |  |  |  |  |  |
Fair value of plan assets at beginning of year |  | 323.1 |  |  | — |  |  | 7.8 |  |  | 458.4 |  |  | — |  |  | 8.0 |
Investment income (loss) |  | 27.4 |  |  | — |  |  | 0.2 |  |  | (98.6 | ) |  | — |  |  | — |
Employer contributions |  | — |  |  | 3.5 |  |  | 4.3 |  |  | — |  |  | 3.1 |  |  | 4.5 |
Retiree contributions |  | — |  |  | — |  |  | 1.1 |  |  | — |  |  | — |  |  | 1.4 |
Benefits paid |  | (41.9 | ) |  | (3.5 | ) |  | (5.4 | ) |  | (36.7 | ) |  | (3.1 | ) |  | (6.1 | )
Fair value of plan assets at end of year |  | 308.6 |  |  | — |  |  | 8.0 |  |  | 323.1 |  |  | — |  |  | 7.8 |
Funded status - deficiency | $ | 39.5 |  | $ | 46.7 |  | $ | 43.1 |  | $ | 35.3 |  | $ | 45.2 |  | $ | 41.9 |
Amounts recognized on our Consolidated Balance Sheets as of December 31: |  |  |  |  |  |  |  |  |  |  |  |
Regulatory assets | $ | 79.9 |  | $ | — |  | $ | 4.8 |  | $ | 78.7 |  | $ | — |  | $ | 3.8 |
Current liabilities |  | — |  |  | 2.4 |  |  | 4.2 |  |  | — |  |  | 2.2 |  |  | 4.4 |
Non-current assets |  | — |  |  | — |  |  | 1.3 |  |  | — |  |  | — |  |  | 1.0 |
Non-current liabilities |  | 39.4 |  |  | 44.3 |  |  | 40.2 |  |  | 35.2 |  |  | 43.0 |  |  | 38.5 |
Regulatory liabilities |  | 2.9 |  |  | — |  |  | 5.5 |  |  | 2.8 |  |  | — |  |  | 6.2 |
Amounts recognized in AOCI, net of tax as of December 31: |  |  |  |  |  |  |  |  |  |  |  |
Net (gain) loss | $ | 5.0 |  | $ | 1.8 |  | $ | (0.7 | ) | $ | 5.2 |  | $ | 1.6 |  | $ | (0.7 | )
Prior service cost (gain) |  | — |  |  | — |  |  | 0.1 |  |  | (0.1 | ) |  | — |  |  | 0.1 |
Total amounts included in AOCI, net of tax not yet recognized as components of net periodic expense | $ | 5.0 |  | $ | 1.8 |  | $ | (0.6 | ) | $ | 5.1 |  | $ | 1.6 |  | $ | (0.6 | )
Components of Net Periodic Benefit Cost | The following table provides a reconciliation of components of the net periodic expense (in millions):
| Defined Benefit Pension Plan |  | Supplemental Non-qualified Defined Benefit Plans |  | Non-pension Defined Benefit Postretirement Healthcare Plan |
For the years ended December 31, | 2023 |  | 2022 |  | 2021 |  | 2023 |  | 2022 |  | 2021 |  | 2023 |  | 2022 |  | 2021 |
Service cost | $ | 2.5 |  | $ | 3.9 |  | $ | 5.0 |  | $ | 3.1 |  | $ | (0.8 | ) | $ | 3.1 |  | $ | 1.5 |  | $ | 1.9 |  | $ | 2.2 |
Interest cost |  | 17.5 |  |  | 10.8 |  |  | 9.3 |  |  | 1.5 |  |  | 0.8 |  |  | 0.7 |  |  | 2.4 |  |  | 1.3 |  |  | 1.0 |
Expected return on assets |  | (18.7 | ) |  | (18.5 | ) |  | (20.8 | ) |  | — |  |  | — |  |  | — |  |  | (0.2 | ) |  | (0.1 | ) |  | (0.1 | )
Net amortization of prior service cost |  | (0.1 | ) |  | (0.1 | ) |  | — |  |  | — |  |  | — |  |  | — |  |  | — |  |  | (0.3 | ) |  | (0.4 | )
Recognized net actuarial loss (gain) |  | 2.0 |  |  | 6.1 |  |  | 7.3 |  |  | — |  |  | 0.3 |  |  | 1.8 |  |  | — |  |  | 0.1 |  |  | 0.5 |
Net periodic expense | $ | 3.2 |  | $ | 2.2 |  | $ | 0.8 |  | $ | 4.6 |  | $ | 0.3 |  | $ | 5.6 |  | $ | 3.7 |  | $ | 2.9 |  | $ | 3.2 |
Schedule of Assumptions Used | Assumptions
| Defined Benefit Pension Plan |  | Supplemental Non-qualified Defined Benefit Plans |  | Non-pension Defined Benefit Postretirement Healthcare Plan |
| 2023 |  | 2022 |  | 2021 |  | 2023 |  | 2022 |  | 2021 |  | 2023 |  | 2022 |  | 2021 |
Weighted-average assumptions used to determine benefit obligations: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Discount rate |  | 4.99 | % |  | 5.17 | % |  | 2.88 | % |  | 4.93 | % |  | 5.13 | % |  | 2.77 | % |  | 4.97 | % |  | 5.14 | % |  | 2.79 | %
Rate of increase in compensation levels |  | 3.04 | % |  | 3.06 | % |  | 3.08 | % |  | — |  |  | — |  |  | 5.00 | % | N/A |  | N/A |  | N/A |
Weighted-average assumptions used to determine net periodic benefit cost for plan year: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Discount rate (a) |  | 5.17 | % |  | 2.88 | % |  | 2.56 | % |  | 5.13 | % |  | 2.77 | % |  | 2.41 | % |  | 5.14 | % |  | 2.79 | % |  | 2.41 | %
Expected long-term rate of return on assets (b) |  | 6.00 | % |  | 4.25 | % |  | 4.50 | % | N/A |  | N/A |  | N/A |  |  | 3.10 | % |  | 1.70 | % |  | 1.80 | %
Rate of increase in compensation levels |  | 3.06 | % |  | 3.08 | % |  | 3.34 | % |  | — |  |  | — |  |  | 5.00 | % | N/A |  | N/A |  | N/A |
(a) The estimated discount rate for the Defined Benefit Pension Plan is 5.0% for the calculation of the 2024 net periodic pension costs. (b) The expected rate of return on plan as sets for the Defined Benefit Pension Plan is 6.0% for the calculation of the 2024 net periodic pension cost.
Schedule of Health Care Cost Trend Rates | The healthcare benefit obligation at December 31 was determined as follows:
| 2023 |  | 2022 |
Trend Rate - Medical |  |  |  |
Pre-65 for next year - All Plans |  | 6.69 | % |  | 7.00 | %
Pre-65 Ultimate trend rate - Black Hills Corp |  | 4.50 | % |  | 4.50 | %
Trend Year | 2034 |  | 2031 |
Post-65 for next year - All Plans |  | 5.81 | % |  | 6.00 | %
Post-65 Ultimate trend rate - Black Hills Corp |  | 4.50 | % |  | 4.50 | %
Trend Year | 2034 |  | 2031 |
Schedule of Expected Benefit Payments | The following benefit payments to employees, which reflect future service, are expected to be paid (in millions):
| Defined Benefit Pension Plan |  | Supplemental Non-qualified Defined Benefit Plans |  | Non-pension Defined Benefit Postretirement Healthcare Plan |
2024 | $ | 24.5 |  | $ | 2.4 |  | $ | 5.2 |
2025 |  | 25.4 |  |  | 2.8 |  |  | 5.0 |
2026 |  | 26.0 |  |  | 2.8 |  |  | 4.9 |
2027 |  | 25.9 |  |  | 2.7 |  |  | 4.8 |
2028 |  | 26.2 |  |  | 2.6 |  |  | 4.6 |
2029 - 2033 | $ | 129.7 |  | $ | 11.7 |  | $ | 21.4 |
Pension Plan |
Defined Benefit Plan Disclosure [Line Items] |
Schedule of Allocation of Plan Assets | The following tables set forth, by level within the fair value hierarchy, the assets that were accounted for at fair value on a recurring basis (in millions):
| December 31, 2023 |
| Level 1 |  | Level 2 |  | Level 3 |  | Total Investments Measured at Fair Value |  | NAV (a) |  | Total Investments |
Pension Plan |  |  |  |  |  |  |  |  |  |  |  |
Common Collective Trust - Cash and Cash Equivalents | $ | — |  | $ | 6.7 |  | $ | — |  | $ | 6.7 |  | $ | — |  | $ | 6.7 |
Common Collective Trust - Equity |  | — |  |  | 42.7 |  |  | — |  |  | 42.7 |  |  | — |  |  | 42.7 |
Common Collective Trust - Fixed Income |  | — |  |  | 234.5 |  |  | — |  |  | 234.5 |  |  | — |  |  | 234.5 |
Common Collective Trust - Real Estate |  | — |  |  | — |  |  | — |  |  | — |  |  | 16.4 |  |  | 16.4 |
Hedge Funds |  | — |  |  | — |  |  | — |  |  | — |  |  | 8.1 |  |  | 8.1 |
Total investments measured at fair value | $ | — |  | $ | 283.9 |  | $ | — |  | $ | 283.9 |  | $ | 24.5 |  | $ | 308.4 |
Non-pension Defined Benefit Postretirement Healthcare Plan |  |  |  |  |  |  |  |  |  |  |  |
Cash and Cash Equivalents |  | 8.0 |  |  | — |  |  | — |  |  | 8.0 |  |  |  |  | 8.0 |
Total investments measured at fair value | $ | 8.0 |  | $ | — |  | $ | — |  | $ | 8.0 |  |  |  | $ | 8.0 |
| December 31, 2022 |
| Level 1 |  | Level 2 |  | Level 3 |  | Total Investments Measured at Fair Value |  | NAV (a) |  | Total Investments |
Pension Plan |  |  |  |  |  |  |  |  |  |  |  |
Common Collective Trust - Cash and Cash Equivalents | $ | — |  | $ | 6.4 |  | $ | — |  | $ | 6.4 |  | $ | — |  | $ | 6.4 |
Common Collective Trust - Equity |  | — |  |  | 45.1 |  |  | — |  |  | 45.1 |  |  | — |  |  | 45.1 |
Common Collective Trust - Fixed Income |  | — |  |  | 242.0 |  |  | — |  |  | 242.0 |  |  | — |  |  | 242.0 |
Common Collective Trust - Real Estate |  | — |  |  | — |  |  | — |  |  | — |  |  | 21.5 |  |  | 21.5 |
Hedge Funds |  | — |  |  | — |  |  | — |  |  | — |  |  | 8.1 |  |  | 8.1 |
Total investments measured at fair value | $ | — |  | $ | 293.5 |  | $ | — |  | $ | 293.5 |  | $ | 29.6 |  | $ | 323.1 |
Non-pension Defined Benefit Postretirement Healthcare Plan |  |  |  |  |  |  |  |  |  |  |  |
Cash and Cash Equivalents |  | 7.8 |  |  | — |  |  | — |  |  | 7.8 |  |  |  |  | 7.8 |
Total investments measured at fair value | $ | 7.8 |  | $ | — |  | $ | — |  | $ | 7.8 |  |  |  | $ | 7.8 |
(a) Certain investments that are measured at fair value using NAV per share (or its equivalent) for practical expedient have not been classified in the fair value hierarchy. The fair value amounts presented in these tables for these investments are intended to permit reconciliation of the fair value hierarchy to the amounts presented in the reconciliation of changes in the plan’s benefit obligations and fair value of plan assets above.

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