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Operations and Significant Accounting Policies - Other Income (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Other Income (Expense) - Other [Line Items]    
Total Other Income $ 3.3 $ 1.0
Pension and Other Postretirement Benefit Plans Non-Service Credit [Member]    
Other Income (Expense) - Other [Line Items]    
Total Other Income [1] 1.9 2.6
Interest and Investment Income (Loss) [Member]    
Other Income (Expense) - Other [Line Items]    
Total Other Income 1.1 (2.6)
AFUDC - Equity [Member]    
Other Income (Expense) - Other [Line Items]    
Total Other Income 0.5 0.5
Gain on Land Sales [Member]    
Other Income (Expense) - Other [Line Items]    
Total Other Income 0.0 0.1
Other [Member]    
Other Income (Expense) - Other [Line Items]    
Total Other Income $ (0.2) $ 0.4
[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.)