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Pension and Postretirement Benefit Costs (Tables)
9 Months Ended
Sep. 30, 2021
Retirement Benefits [Abstract]  
Schedule of Net Benefit Costs
Net periodic pension benefit included the following components:
Three Months Ended September 30,Nine Months Ended September 30,
2021202020212020
 (Dollars in millions)
Service cost for benefits earned$— $0.1 $0.1 $0.2 
Interest cost on projected benefit obligation5.2 7.0 15.4 21.0 
Expected return on plan assets(5.8)(7.5)(17.2)(22.3)
Net periodic pension benefit$(0.6)$(0.4)$(1.7)$(1.1)
Net periodic postretirement benefit (benefit) cost included the following components:
Three Months Ended September 30,Nine Months Ended September 30,
2021202020212020
 (Dollars in millions)
Service cost for benefits earned$0.3 $1.1 $0.8 $3.3 
Interest cost on accumulated postretirement benefit obligation2.9 5.4 8.7 16.3 
Expected return on plan assets(0.3)(0.3)(0.7)(1.1)
Amortization of prior service credit(11.0)(2.2)(33.0)(6.6)
Net actuarial loss— 13.0 — 13.0 
Net periodic postretirement benefit (benefit) cost$(8.1)$17.0 $(24.2)$24.9