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Pension and Other Postretirement Benefit Plans
 9 Months Ended
Sep. 30, 2023
Retirement Benefits [Abstract]  
Pension and Other Postretirement Benefit Plans PENSION AND OTHER POSTRETIREMENT BENEFIT PLANS
 PensionOther
Postretirement
Components of Net Periodic Benefit Cost (Credit)2023202220232022
Millions    
Quarter Ended September 30,    
Service Cost$1.7 $2.1 $0.6 $0.8 
Non-Service Cost Components (a)
Interest Cost10.2 6.9 1.6 1.1 
Expected Return on Plan Assets(10.9)(10.4)(2.9)(2.4)
Amortization of Prior Service Credits(0.1)— (1.8)(1.8)
Amortization of Net Loss1.4 2.8 (0.6)0.1 
Net Periodic Benefit Cost (Credit)$2.3 $1.4 $(3.1)$(2.2)
Nine Months Ended September 30,
Service Cost$4.9 $6.9 $1.7 $2.3 
Non-Service Cost Components (a)
Interest Cost 30.4 20.4 4.6 3.3 
Expected Return on Plan Assets (32.8)(31.1)(8.5)(7.2)
Amortization of Prior Service Credits (0.1)(0.1)(5.3)(5.6)
Amortization of Net Loss4.3 8.6 (1.7)0.3 
Net Periodic Benefit Cost (Credit)$6.7 $4.7 $(9.2)$(6.9)
(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 nine months ended September 30, 2023, we contributed $17.3 million in cash to the defined benefit pension plans (none for the nine months ended September 30, 2022); we do not expect to make additional contributions to our defined benefit pension plans in 2023. For the nine months ended September 30, 2023 and 2022, 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 2023.