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Pension and Postretirement Benefit Costs (Tables)
6 Months Ended
Jun. 30, 2021
Retirement Benefits [Abstract]  
Schedule of Net Benefit Costs
Net periodic pension benefit included the following components:
Three Months Ended June 30,Six Months Ended June 30,
2021202020212020
 (Dollars in millions)
Service cost for benefits earned$0.1 $0.1 $0.1 $0.1 
Interest cost on projected benefit obligation5.1 7.0 10.2 14.0 
Expected return on plan assets(5.7)(7.4)(11.4)(14.8)
Net periodic pension benefit$(0.5)$(0.3)$(1.1)$(0.7)
Net periodic postretirement benefit (benefit) cost included the following components:
Three Months Ended June 30,Six Months Ended June 30,
2021202020212020
 (Dollars in millions)
Service cost for benefits earned$0.3 $1.1 $0.5 $2.2 
Interest cost on accumulated postretirement benefit obligation2.9 5.4 5.8 10.9 
Expected return on plan assets(0.2)(0.4)(0.4)(0.8)
Amortization of prior service credit(11.0)(2.2)(22.0)(4.4)
Net periodic postretirement benefit (benefit) cost$(8.0)$3.9 $(16.1)$7.9