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Components of Gas Inventory (Detail) - USD ($)
$ in Millions
Jun. 30, 2018
Dec. 31, 2017
Jun. 30, 2017
Public Utilities, Inventory [Line Items]      
Weighted average cost inventory amount $ 0.6 $ 0.6 $ 0.5
Liquefied Natural Gas & Other      
Public Utilities, Inventory [Line Items]      
Weighted average cost inventory amount 0.1 0.1 0.1
Natural Gas      
Public Utilities, Inventory [Line Items]      
Weighted average cost inventory amount 0.2 0.4 0.2
Propane      
Public Utilities, Inventory [Line Items]      
Weighted average cost inventory amount $ 0.3 $ 0.1 $ 0.2