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Pension and Postretirement Benefit Costs (Tables)
 9 Months Ended
Sep. 30, 2024
Retirement Benefits [Abstract]  
Schedule of Net Benefit Costs
Net periodic pension cost included the following components:
Three Months Ended September 30,Nine Months Ended September 30,
2024202320242023
 (Dollars in millions)
Service cost for benefits earned$0.1 $0.1 $0.1 $0.1 
Interest cost on projected benefit obligation1.7 3.6 4.9 18.4 
Expected return on plan assets(1.3)(3.1)(3.7)(16.3)
Net periodic pension cost$0.5 $0.6 $1.3 $2.2 
Net periodic postretirement benefit credit included the following components:
Three Months Ended September 30,Nine Months Ended September 30,
2024202320242023
 (Dollars in millions)
Service cost for benefits earned$0.1 $0.1 $0.3 $0.4 
Interest cost on accumulated postretirement benefit obligation2.3 2.6 6.9 7.6 
Expected return on plan assets(0.1)(0.1)(0.3)(0.4)
Amortization of prior service credit(13.3)(13.5)(39.8)(40.3)
Net periodic postretirement benefit credit$(11.0)$(10.9)$(32.9)$(32.7)