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Pension and Postretirement Benefit Costs (Tables)
3 Months Ended
Mar. 31, 2025
Retirement Benefits [Abstract]  
Schedule of Net Benefit Costs
Net periodic postretirement benefit credit included the following components:
Three Months Ended March 31,
20252024
 (Dollars in millions)
Service cost for benefits earned$0.1 $0.1 
Interest cost on accumulated postretirement benefit obligation2.0 2.3 
Expected return on plan assets(0.1)(0.1)
Amortization of prior service credit(10.2)(13.2)
Net periodic postretirement benefit credit$(8.2)$(10.9)