Operations and Significant Accounting Policies - Other Income (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Other Income (Expense) - Other [Line Items]
Total Other Income		$ 1.0	$ 2.9	$ 6.1	$ 9.1
Pension and Other Postretirement Benefit Plans Non-Service Credit [Member]
Other Income (Expense) - Other [Line Items]
Total Other Income	[1]			4.4	6.5
Interest and Investment Income (Loss) [Member]
Other Income (Expense) - Other [Line Items]
Total Other Income				1.7	0.4
AFUDC - Equity [Member]
Other Income (Expense) - Other [Line Items]
Total Other Income				1.7	1.8
Other [Member]
Other Income (Expense) - Other [Line Items]
Total Other Income				$ (1.7)	$ 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.)