Regulatory Liabilities (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2017
Regulatory Liabilities [Line Items]
Regulatory Liabilities	$ 49.8	$ 54.0	$ 48.9
Less: Current Portion of Regulatory Liabilities	5.5	7.4
Regulatory Liabilities-noncurrent	44.3	46.6
Rate Adjustment Mechanisms
Regulatory Liabilities [Line Items]
Regulatory Liabilities	4.1	6.0
Income Taxes
Regulatory Liabilities [Line Items]
Regulatory Liabilities	45.5	47.6
Other
Regulatory Liabilities [Line Items]
Regulatory Liabilities	$ 0.2	$ 0.4