Operations and Significant Accounting Policies - Other Income (Expense) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Other Income (Expense) - Other [Line Items]
Total Other Income (Expense) - Other	$ 85.0	$ 22.4	$ 8.7
Pension and Other Postretirement Benefit Plan Non-Service Credit
Other Income (Expense) - Other [Line Items]
Total Other Income (Expense) - Other	8.9	9.8	6.1
Interest and Investment Income
Other Income (Expense) - Other [Line Items]
Total Other Income (Expense) - Other	10.3	0.0	2.3
AFUDC - Equity
Other Income (Expense) - Other [Line Items]
Total Other Income (Expense) - Other	3.6	2.7	2.6
Gain on Land Sales
Other Income (Expense) - Other [Line Items]
Total Other Income (Expense) - Other	0.2	0.0	0.1
PSA Liability
Other Income (Expense) - Other [Line Items]
Total Other Income (Expense) - Other	0.0	10.2	0.0
Gain on Arbitration Award
Other Income (Expense) - Other [Line Items]
Total Other Income (Expense) - Other	58.4	0.0	0.0
Other
Other Income (Expense) - Other [Line Items]
Total Other Income (Expense) - Other	3.6	$ (0.3)	$ (2.4)
Interest Income Related To Interest Awarded As Part Of An Arbitration Ruling
Other Income (Expense) - Other [Line Items]
Total Other Income (Expense) - Other	$ 5.1