Regulatory Liabilities (Detail) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Regulatory Liabilities [Line Items]
Regulatory Liabilities	$ 13.0	$ 23.7
Less: Current Portion of Regulatory Liabilities	10.4	15.6
Regulatory Liabilities-noncurrent	2.6	8.1
Regulatory Tracker Mechanisms
Regulatory Liabilities [Line Items]
Regulatory Liabilities	6.2	8.0
Gas Pipeline Refund
Regulatory Liabilities [Line Items]
Regulatory Liabilities	$ 6.8	$ 15.7