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Components of Net Periodic Benefit Cost
 9 Months Ended
Sep. 30, 2020
Components of net periodic benefit cost [Abstract]  
Components of Net Periodic Benefit Cost Components of Net Periodic Benefit Cost
Pension Benefits
Three months ended September 30,
Nine months ended September 30,
2020
2019
2020
2019
Service cost$1.3 $1.0 $3.7 $2.5 
Interest cost2.9 3.3 8.5 10.0 
Expected return on plan assets(5.3)(4.5)(15.6)(13.7)
Amortization of prior service costs0.1 0.0 0.1 0.6 
Amortization of net loss1.6 1.6 4.9 4.9 
Net periodic benefit cost$0.6 $1.4 $1.6 $4.3 
Other Postretirement Benefits ("OPEB")
Three months ended September 30,
Nine months ended September 30,
2020
2019
2020
2019
Service cost$0.0 $0.0 $0.2 $0.8 
Interest cost0.8 1.1 2.3 3.4 
Amortization of prior service cost(0.8)(1.2)(2.4)(4.3)
Amortization of net loss0.5 0.6 1.6 1.7 
Net periodic benefit cost$0.5 $0.5 $1.7 $1.6