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Components of Gas Inventory (Detail) - USD ($)
$ in Millions
Sep. 30, 2019
Dec. 31, 2018
Sep. 30, 2018
Public Utilities, Inventory [Line Items]      
Weighted average cost inventory amount $ 0.8 $ 0.8 $ 0.9
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 0.4 0.3 0.4
Propane      
Public Utilities, Inventory [Line Items]      
Weighted average cost inventory amount $ 0.3 $ 0.4 $ 0.4