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Postretirement Benefits Other Than Pensions - Components Of Comprehensive OPEB (Income) Expense (Details) - Other Postretirement Benefit Plan, Defined Benefit - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]        
Service Cost Earned During the Year $ 0.3 $ 0.2 $ 0.2  
Interest Cost on Accumulated Postretirement Benefit Obligation 0.1 0.3 0.4  
Amortization of Prior Service Credit (1.3) (1.3) (1.3)  
Amortization of Accumulated Unrecognized OPEB Gain (1.7) (1.9) (2.4)  
Pension Expense (Income) Recognized in Consolidated Statements of Income (2.6) (2.7) (3.1)  
Unrecognized OPEB (Gain) Loss Arising During the Year (1.8) 1.9 (1.7)  
Prior Service Credit Arising During the Year from Plan Amendments       $ 11.0
Amortization of Prior Service Credit 1.3 1.3 1.3  
Amortization of Accumulated Unrecognized OPEB Gain 1.7 1.9 2.4  
Comprehensive OPEB (Income) Expense $ (1.4) $ 2.4 $ (1.1)