Summary of Interest Expense and Interest Income (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Interest Expense
Long-Term Debt		$ 26.0	$ 24.8	$ 22.9
Short-Term Debt		0.8	1.4	3.0
Regulatory Liabilities		0.4	0.2	0.7
Subtotal Interest Expense		27.2	26.4	26.6
Interest Income
Interest and Other Income		(1.6)	(2.6)	(2.9)
Total Interest Expense, Net		25.6	23.8	23.7
Regulatory Assets
Interest Income
Interest and Other Income		(0.5)	(0.8)	(0.8)
AFUDC and Other
Interest Income
Interest and Other Income	[1]	$ (1.1)	$ (1.8)	$ (2.1)

[1] AFUDC—Allowance for Funds Used During Construction