Postretirement Benefits Other Than Pensions (Tables) - Other Postretirement Benefit Plan, Defined Benefit

12 Months Ended

Dec. 31, 2023

Defined Benefit Plan Disclosure [Line Items]

Schedule of Changes in Fair Value of Plan Assets

Changes in Fair Value of Plans’ Assets and Changes in Accumulated Postretirement Benefit Obligation for the years ended December 31, 2023 and 2022 were:

DOLLARS IN MILLIONS
	2023		2022
Fair Value of Plans’ Assets at Beginning of Year	$	—	$	—
Employer Contributions	1.0		1.0
Plan Participants’ Contributions	0.3		0.3
Benefits Paid	(1.3)		(1.3)
Fair Value of Plan Assets at End of Year	—		—
Accumulated Postretirement Benefit Obligation at Beginning of Year	8.1		11.2
Service Cost	0.1		0.2
Interest Cost	0.4		0.2
Plan Participants’ Contributions	0.3		0.3
Benefits Paid	(1.3)		(1.3)
Actuarial Gain	(0.1)		(2.5)
Accumulated Postretirement Benefit Obligation at End of Year	7.5		8.1
Funded Status—Accumulated Postretirement Benefit Obligation in Excess of Plans’ Assets	$	(7.5)	$	(8.1)
Unamortized Actuarial Gain Reported in AOCI at End of Year	$	13.9	$	16.9

Schedule of Assumptions Used

The weighted-average discount rate and rate of increase in future compensation levels used to develop the components of the Accumulated Postretirement Benefit Obligation at December 31, 2023 and 2022 were:

	2023		2022
Discount Rate	4.92	%	5.08	%
Rate of Increase in Future Compensation Levels	2.20		2.20

The weighted-average discount rate and rate of increase in future compensation levels used to develop OPEB Expense for the years ended December 31, 2023, 2022 and 2021 were:

	2023		2022		2021
Weighted-average Discount Rate	5.11	%	2.56	%	1.99	%
Service Cost Discount Rate	5.12		2.79		2.06
Interest Cost Discount Rate	5.03		1.97		1.19
Effective Rate for Interest on Service Cost	5.04		2.54		—
Rate of Increase in Future Compensation Levels	2.20		2.20		2.20

Schedule of Comprehensive Other Postretirement Employee Benefit Expense

The components of Comprehensive OPEB (Income) Expense for the years ended December 31, 2023, 2022 and 2021 were:

DOLLARS IN MILLIONS	2023		2022		2021
Service Cost Earned During the Year	$	0.1	$	0.2	$	0.3
Interest Cost on Accumulated Postretirement Benefit Obligation	0.4		0.2		0.1
Amortization of Prior Service Credit	(1.3)		(1.3)		(1.3)
Amortization of Accumulated Unrecognized OPEB Gain	(1.8)		(1.8)		(1.7)
OPEB Income Recognized in Consolidated Statements of Loss	(2.6)		(2.7)		(2.6)
Unrecognized OPEB Gain Arising During the Year	(0.1)		(2.5)		(1.8)
Amortization of Prior Service Credit	1.3		1.3		1.3
Amortization of Accumulated Unrecognized OPEB Gain	1.8		1.8		1.7
Comprehensive OPEB Expense (Income)	$	0.4	$	(2.1)	$	(1.4)

Schedule of Expected Benefit Payments

The following benefit payments (net of participant contributions), which consider expected future service, as appropriate, are expected to be paid:

DOLLARS IN MILLIONS	Years Ending December 31,
	2024		2025		2026		2027		2028		2029-2032
Estimated Benefit Payments:
Excluding Medicare Part D Subsidy	$	0.9	$	0.9	$	0.9	$	0.8	$	0.7	$	3.0
Expected Medicare Part D Subsidy	—		—		—		—		—		—
Net Estimated Benefit Payments	$	0.9	$	0.9	$	0.9	$	0.8	$	0.7	$	3.0