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Components of Net Periodic Benefit Cost
 6 Months Ended
Jun. 30, 2021
Retirement Benefits [Abstract]  
Components of Net Periodic Benefit Cost Components of Net Periodic Benefit Cost
Pension Benefits
Three months ended June 30,
Six months ended June 30,
2021
2020
20212020
Service cost$0.9 $1.1 $2.3 $2.4 
Interest cost2.4 $2.8 4.8 5.6 
Expected return on plan assets(5.6)$(5.2)(11.1)(10.3)
Amortization of prior service costs0.1 0.00.1 0.0
Amortization of net loss1.5 $1.8 3.0 3.3 
Net periodic benefit cost$(0.7)$0.5 $(0.9)$1.0 
Other Postretirement Benefits ("OPEB")
Three months ended June 30,
Six months ended June 30,
2021
2020
20212020
Service cost$0.0 $0.1 $0.1 $0.2 
Interest cost0.6 0.7 1.2 1.5 
Amortization of prior service cost(0.8)(0.8)(1.6)(1.6)
Amortization of net loss0.5 0.1 1.1 1.1 
Net periodic benefit cost$0.3 $0.1 $0.8 $1.2