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Components of Net Periodic Benefit Cost
 9 Months Ended
Sep. 30, 2022
Retirement Benefits [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,
2022202120222021
Service cost$1.1 $1.2 $3.2 $3.5 
Interest cost2.6 2.4 7.7 7.3 
Expected return on plan assets(5.9)(5.6)(17.6)(16.8)
Amortization of prior service costs0.0 0.00.1 0.1 
Amortization of net loss0.9 1.5 2.6 4.5 
Net periodic benefit cost (income)$(1.3)$(0.5)$(4.0)$(1.4)
Other Postretirement Benefits
Three months ended September 30,Nine months ended September 30,
2022202120222021
Service cost$0.0 $0.1 $0.1 $0.2 
Interest cost0.9 0.6 2.2 1.8 
Amortization of prior service cost(0.3)(0.8)(1.3)(2.4)
Amortization of net loss0.2 0.6 1.0 1.7 
Curtailment gain, net(8.0)— (8.0)— 
Net periodic benefit cost (income)$(7.2)$0.5 $(6.0)$1.3 
We measure our obligations under our defined benefit plan and other post retirement benefits annually or more frequently at interim periods when a significant event occurs. ASC 715, Compensation-Retirement Benefits, defines a curtailment as an event that significantly reduces the expected years of future service of present employees or eliminates defined benefits for some or all of a significant number of employees. In the third quarter, we took actions to re-measure certain other postretirement benefits triggered by the Hawesville smelter curtailment, leading to a non-cash OPEB curtailment gain, net totaling $8.0 million for the three and nine months ended September 30, 2022.