Regulatory Liabilities (Detail) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Regulatory Liabilities [Line Items]
Regulatory Liabilities	$ 60.0	$ 58.1	$ 13.5
Less: Current Portion of Regulatory Liabilities	10.9	9.2	12.8
Regulatory Liabilities-noncurrent	49.1	48.9	0.7
Rate Adjustment Mechanisms
Regulatory Liabilities [Line Items]
Regulatory Liabilities	10.3	6.9	9.2
Gas Pipeline Refund
Regulatory Liabilities [Line Items]
Regulatory Liabilities	0.6	2.3	$ 4.3
Income Tax Related Liabilities
Regulatory Liabilities [Line Items]
Regulatory Liabilities	$ 49.1	$ 48.9