Summary of Interest Expense and Interest Income (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Interest Expense
Long-Term Debt		$ 21.8	$ 21.8	$ 22.0
Short-Term Debt		2.5	1.4	0.9
Regulatory Liabilities		1.2	0.5	0.9
Interest Expense		25.5	23.7	23.8
Interest Income
Interest income		(2.4)	(1.2)	(1.9)
Total Interest Expense, net		23.1	22.5	21.9
Regulatory Assets
Interest Income
Interest income		(0.7)	(0.3)	(0.7)
AFUDC and Other
Interest Income
Interest income	[1]	$ (1.7)	$ (0.9)	$ (1.2)

[1] AFUDC-Allowance for Funds Used During Construction