Pension and Postretirement Benefit Costs (Tables)

6 Months Ended

Jun. 30, 2023

Retirement Benefits [Abstract]

Schedule of Net Benefit Costs

Net periodic pension cost (credit) included the following components:

	Three Months Ended June 30,				Six Months Ended June 30,
	2023		2022		2023		2022
	(Dollars in millions)
Interest cost on projected benefit obligation	$	7.4	$	5.4	$	14.8	$	10.7
Expected return on plan assets	(6.6)		(6.0)		(13.2)		(11.9)
Net periodic pension cost (credit)	$	0.8	$	(0.6)	$	1.6	$	(1.2)

Net periodic postretirement benefit credit included the following components:

	Three Months Ended June 30,				Six Months Ended June 30,
	2023		2022		2023		2022
	(Dollars in millions)
Service cost for benefits earned	$	0.2	$	0.2	$	0.3	$	0.4
Interest cost on accumulated postretirement benefit obligation	2.5		1.8		5.0		3.5
Expected return on plan assets	(0.2)		(0.2)		(0.3)		(0.4)
Amortization of prior service credit	(13.4)		(13.5)		(26.8)		(26.9)
Net periodic postretirement benefit credit	$	(10.9)	$	(11.7)	$	(21.8)	$	(23.4)