Operations and Significant Accounting Policies - Other Income (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022		Sep. 30, 2021
Other Income (Expense) - Other [Line Items]
Total Other Income		$ 2.3	$ 1.0	$ 16.4		$ 6.1
Pension and Other Postretirement Benefit Plans Non-Service Credit [Member]
Other Income (Expense) - Other [Line Items]
Total Other Income	[1]	1.9	1.6	7.2		4.4
Interest and Investment Income (Loss) [Member]
Other Income (Expense) - Other [Line Items]
Total Other Income		(0.4)	(0.1)	(1.3)		1.7
AFUDC - Equity [Member]
Other Income (Expense) - Other [Line Items]
Total Other Income		0.7	0.6	2.4		1.7
PSA Liability [Member]
Other Income (Expense) - Other [Line Items]
Total Other Income		0.0	0.0	10.2	[2]	0.0
Other [Member]
Other Income (Expense) - Other [Line Items]
Total Other Income		$ 0.1	$ (1.1)	$ (2.1)		$ (1.7)

[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.