Operations and Significant Accounting Policies - Other Income (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Other Income (Expense) - Other [Line Items]
Total Other Income		$ 1.8	$ 5.2	$ 5.1	$ 6.2
Pension and Other Postretirement Benefit Plans Non-Service Credit [Member]
Other Income (Expense) - Other [Line Items]
Total Other Income	[1]			2.8	4.8
Interest and Investment Income (Loss) [Member]
Other Income (Expense) - Other [Line Items]
Total Other Income				1.8	(0.5)
AFUDC - Equity [Member]
Other Income (Expense) - Other [Line Items]
Total Other Income				1.1	1.4
Other [Member]
Other Income (Expense) - Other [Line Items]
Total Other Income				$ (0.6)	$ 0.5

[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.)