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Components of Net Periodic Benefit Cost
 6 Months Ended
Jun. 30, 2025
Retirement Benefits [Abstract]  
Components of Net Periodic Benefit Cost
7.    Components of Net Periodic Benefit Cost
Pension Benefits
Three months ended June 30,Six months ended
June 30,
2025202420252024
Service cost$0.6 $0.7 $1.3 $1.3 
Interest cost3.5 3.2 6.8 6.7 
Expected return on plan assets(4.0)(4.1)(7.9)(7.9)
Amortization of prior service benefit— — 0.1 0.1 
Amortization of net loss1.5 1.3 2.9 3.0 
Net periodic benefit cost$1.6 $1.1 $3.2 $3.2 
Other Postretirement Benefits
Three months ended June 30,Six months ended
June 30,
2025202420252024
Service cost$0.1 $0.1 $0.1 $0.1 
Interest cost0.9 0.9 1.9 1.9 
Amortization of prior service cost— (0.1)— (0.1)
Amortization of net loss0.1 0.3 0.3 0.3 
Net periodic benefit cost$1.1 $1.2 $2.3 $2.2