Operations and Significant Accounting Policies - Other Income (Expense) - Other (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Other Income (Expense) - Other [Line Items]
Total Other Income (Expense) - Other		$ 18.7	$ 7.8	$ 6.3
Pension and Other Postretirement Benefit Plans Non-Service Credit [Member]
Other Income (Expense) - Other [Line Items]
Total Other Income (Expense) - Other	[1]	7.7	4.6	3.9
Interest and Investment Earnings [Member]
Other Income (Expense) - Other [Line Items]
Total Other Income (Expense) - Other		4.4	0.5	1.8
AFUDC - Equity [Member]
Other Income (Expense) - Other [Line Items]
Total Other Income (Expense) - Other		2.3	1.2	1.2
Gain (Loss) on Land Sales [Member]
Other Income (Expense) - Other [Line Items]
Total Other Income (Expense) - Other		2.1	0.9	(0.5)
Other Income (Expense) - Other [Member]
Other Income (Expense) - Other [Line Items]
Total Other Income (Expense) - Other		$ 2.2	$ 0.6	$ (0.1)

[1] (a) These are components of net periodic pension and other postretirement benefit cost other than service cost. (See Note 12. Pension and Other Postretirement Benefit Plans.)