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Pension and Other Postretirement Benefit Plans
 3 Months Ended
Mar. 31, 2024
Retirement Benefits [Abstract]  
Pension and Other Postretirement Benefit Plans PENSION AND OTHER POSTRETIREMENT BENEFIT PLANS
Components of Net Periodic Benefit Cost (Credit)PensionOther
Postretirement
Three Months Ended March 31,2024202320242023
Millions    
Service Cost$1.6 $1.6 $0.4 $0.6 
Non-Service Cost Components (a)
Interest Cost 9.7 10.1 1.0 1.5 
Expected Return on Plan Assets (11.2)(10.9)(2.8)(2.8)
Amortization of Prior Service Credits — — (2.9)(1.8)
Amortization of Net Loss1.6 1.4 (0.8)(0.5)
Net Periodic Benefit Cost (Credit)$1.7 $2.2 $(5.1)$(3.0)
(a)These components of net periodic benefit cost (credit) are included in the line item “Other” under Other Income (Expense) on the Consolidated Statement of Income.

Employer Contributions. For the three months ended March 31, 2024, we contributed $25.0 million in cash to the defined benefit pension plans ($6.5 million for the three months ended March 31, 2023); we do not expect to make additional contributions to our defined benefit pension plans in 2024. For the three months ended March 31, 2024 and 2023, we made no contributions to our other postretirement benefit plans; we do not expect to make any contributions to our other postretirement benefit plans in 2024.