Operations and Significant Accounting Policies - Other Income (Expense) - Other (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Other Income (Expense) - Other [Line Items]
Total Other Income (Expense) - Other		$ 14.7	$ 18.7	$ 7.8
Pension and Other Postretirement Benefit Plans Non-Service Credit [Member]
Other Income (Expense) - Other [Line Items]
Total Other Income (Expense) - Other	[1]	8.6	7.7	4.6
Interest and Investment Earnings [Member]
Other Income (Expense) - Other [Line Items]
Total Other Income (Expense) - Other		1.6	4.4	0.5
AFUDC - Equity [Member]
Other Income (Expense) - Other [Line Items]
Total Other Income (Expense) - Other		1.9	2.3	1.2
Gain (Loss) on Land Sales [Member]
Other Income (Expense) - Other [Line Items]
Total Other Income (Expense) - Other		0.4	2.1	0.9
Other Income (Expense) - Other [Member]
Other Income (Expense) - Other [Line Items]
Total Other Income (Expense) - Other		$ 2.2	$ 2.2	$ 0.6

[1] These are components of net periodic pension and other postretirement benefit cost other than service cost. (See Note 11. Pension and Other Postretirement Benefit Plans.