Regulatory Liabilities (Detail) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2017
Regulatory Liability [Line Items]
Regulatory Liabilities	$ 51.9	$ 52.1
Less: Current Portion of Regulatory Liabilities	15.0	9.5
Regulatory Liabilities-noncurrent	36.9	42.6
Rate Adjustment Mechanisms
Regulatory Liability [Line Items]
Regulatory Liabilities	10.9	7.7
Income Taxes
Regulatory Liability [Line Items]
Regulatory Liabilities	41.0	44.3	$ 48.9
Other
Regulatory Liability [Line Items]
Regulatory Liabilities	$ 0.0	$ 0.1