Regulatory Liabilities (Detail) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2017
Regulatory Liabilities [Line Items]
Regulatory Liabilities	$ 52.1	$ 49.8
Less: Current Portion of Regulatory Liabilities	9.5	5.5
Regulatory Liabilities-noncurrent	42.6	44.3
Rate Adjustment Mechanisms
Regulatory Liabilities [Line Items]
Regulatory Liabilities	7.7	4.1
Income Taxes
Regulatory Liabilities [Line Items]
Regulatory Liabilities	44.3	45.5	$ 48.9
Other
Regulatory Liabilities [Line Items]
Regulatory Liabilities	$ 0.1	$ 0.2