Operations and Significant Accounting Policies - Other Income (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2020	Mar. 31, 2019
Other Income (Expense) - Other [Line Items]
Total Other Income		$ 1.0	$ 7.4
Pension and Other Postretirement Benefit Plans Non-Service Credit [Member]
Other Income (Expense) - Other [Line Items]
Total Other Income	[1]	2.6	2.0
Interest and Investment Income (Loss) [Member]
Other Income (Expense) - Other [Line Items]
Total Other Income		(2.6)	1.0
AFUDC - Equity [Member]
Other Income (Expense) - Other [Line Items]
Total Other Income		0.5	0.6
Gain on Land Sales [Member]
Other Income (Expense) - Other [Line Items]
Total Other Income		0.1	1.8
Other [Member]
Other Income (Expense) - Other [Line Items]
Total Other Income		$ 0.4	$ 2.0

[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.)