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Pension and Other Postretirement Benefit Plans (Tables)
3 Months Ended
Mar. 31, 2021
Retirement Benefits [Abstract]  
Components of Net Periodic Benefit Expense (Income) [Table Text Block]
 PensionOther
Postretirement
Components of Net Periodic Benefit Cost (Credit)2021202020212020
Millions    
Three Months Ended March 31,
Service Cost$2.7 $2.6 $0.9 $0.8 
Non-Service Cost Components (a)
Interest Cost 6.1 7.0 1.1 1.3 
Expected Return on Plan Assets (10.8)(10.7)(2.4)(2.5)
Amortization of Prior Service Credits — — (1.9)(2.0)
Amortization of Net Loss4.7 3.2 0.7 0.3 
Net Periodic Benefit Cost (Credit)$2.7 $2.1 $(1.6)$(2.1)
(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.