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Employee Benefit Plans (Tables)

12 Months Ended

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 assets 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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