Operations and Significant Accounting Policies - Other Income (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Other Income (Expense) - Other [Line Items]
Total Other Income		$ 5.2	$ 4.2	$ 6.2	$ 11.6
Pension and Other Postretirement Benefit Plans Non-Service Credit [Member]
Other Income (Expense) - Other [Line Items]
Total Other Income	[1]			4.8	3.6
Interest and Investment Income (Loss) [Member]
Other Income (Expense) - Other [Line Items]
Total Other Income				(0.5)	2.5
AFUDC - Equity [Member]
Other Income (Expense) - Other [Line Items]
Total Other Income				1.4	1.3
Gain on Land Sales [Member]
Other Income (Expense) - Other [Line Items]
Total Other Income				0.1	2.0
Other [Member]
Other Income (Expense) - Other [Line Items]
Total Other Income				$ 0.4	$ 2.2

[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.)