Operations and Significant Accounting Policies - Other Income (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Other Income (Expense) - Other [Line Items]
Total Other Income		$ 2.9	$ 3.0	$ 9.1	$ 14.6
Pension and Other Postretirement Benefit Plans Non-Service Credit [Member]
Other Income (Expense) - Other [Line Items]
Total Other Income	[1]			6.5	5.2
Interest and Investment Income (Loss) [Member]
Other Income (Expense) - Other [Line Items]
Total Other Income				0.4	3.3
AFUDC - Equity [Member]
Other Income (Expense) - Other [Line Items]
Total Other Income				1.8	1.9
Gain on Land Sales [Member]
Other Income (Expense) - Other [Line Items]
Total Other Income				0.5	2.1
Other [Member]
Other Income (Expense) - Other [Line Items]
Total Other Income				$ (0.1)	$ 2.1

[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.)