Regulatory Liabilities (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Regulatory Liabilities [Line Items]
Regulatory Liabilities	$ 58.1	$ 13.0
Less: Current Portion of Regulatory Liabilities	9.2	10.4
Regulatory Liabilities-noncurrent	48.9	2.6
Rate Adjustment Mechanisms
Regulatory Liabilities [Line Items]
Regulatory Liabilities	6.9	6.2
Gas Pipeline Refund
Regulatory Liabilities [Line Items]
Regulatory Liabilities	2.3	$ 6.8
Income Tax Related Liabilities
Regulatory Liabilities [Line Items]
Regulatory Liabilities	$ 48.9