Components of Gas Inventory (Detail) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Public Utilities, Inventory [Line Items]
Weighted average cost inventory amount	$ 0.6	$ 0.8
Liquefied Natural Gas & Other
Public Utilities, Inventory [Line Items]
Weighted average cost inventory amount	0.1	0.2
Natural Gas
Public Utilities, Inventory [Line Items]
Weighted average cost inventory amount	0.3	0.3
Propane
Public Utilities, Inventory [Line Items]
Weighted average cost inventory amount	$ 0.2	$ 0.3