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Operations and Significant Accounting Policies - Other Income (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2023
Mar. 31, 2022
Other Income (Expense) - Other [Line Items]    
Total Other Income $ 4.1 $ 2.0
Pension and Other Postretirement Benefit Plans Non-Service Credit [Member]    
Other Income (Expense) - Other [Line Items]    
Total Other Income [1] 2.0 2.6
Interest and Investment Income (Loss) [Member]    
Other Income (Expense) - Other [Line Items]    
Total Other Income 1.1 0.1
AFUDC - Equity [Member]    
Other Income (Expense) - Other [Line Items]    
Total Other Income 0.5 0.9
Other [Member]    
Other Income (Expense) - Other [Line Items]    
Total Other Income $ 0.5 $ (1.6)
[1] These are components of net periodic pension and other postretirement benefit cost other than service cost. (See Note 9. Pension and Other Postretirement Benefit Plans.)