Summary of Interest Expense and Interest Income (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Interest Expense
Long-Term Debt		$ 24.8	$ 22.9	$ 23.1
Short-Term Debt		1.4	3.0	2.6
Regulatory Liabilities		0.2	0.7	0.7
Interest Expense		26.4	26.6	26.4
Interest Income
Interest and Other Income		(2.6)	(2.9)	(2.4)
Total Interest Expense, Net		23.8	23.7	24.0
Regulatory Assets
Interest Income
Interest and Other Income		(0.8)	(0.8)	(0.8)
AFUDC and Other
Interest Income
Interest and Other Income	[1]	$ (1.8)	$ (2.1)	$ (1.6)

[1] AFUDC—Allowance for Funds Used During Construction