Operations and Significant Accounting Policies - Other Income (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Other Income (Expense) - Other [Line Items]
Total Other Income		$ 12.1	$ 1.8	$ 14.1	$ 5.1
Pension and Other Postretirement Benefit Plans Non-Service Credit [Member]
Other Income (Expense) - Other [Line Items]
Total Other Income	[1]			5.3	2.8
Interest and Investment Income (Loss) [Member]
Other Income (Expense) - Other [Line Items]
Total Other Income				(0.9)	1.8
AFUDC - Equity [Member]
Other Income (Expense) - Other [Line Items]
Total Other Income				1.7	1.1
PSA Liability
Other Income (Expense) - Other [Line Items]
Total Other Income	[2]			10.2	0.0
Other [Member]
Other Income (Expense) - Other [Line Items]
Total Other Income				$ (2.2)	$ (0.6)

[1] These are components of net periodic pension and other postretirement benefit cost other than service cost. (See Note 10. Pension and Other Postretirement Benefit Plans.)

[2] The gain on removal of the PSA liability for the Northern Wind project upon decommissioning of the legacy wind energy facility assets, which was fully offset by a reserve for an anticipated loss on the sale of the Northern Wind project.