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Components of Gas Inventory (Detail) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2013
Dec. 31, 2012
Mar. 31, 2012
Public Utilities, Inventory [Line Items]      
Weighted average cost inventory amount $ 0.5 $ 1.1 $ 0.6
Natural Gas
     
Public Utilities, Inventory [Line Items]      
Weighted average cost inventory amount   0.6 0.1
Propane
     
Public Utilities, Inventory [Line Items]      
Weighted average cost inventory amount 0.3 0.4 0.4
Liquefied Natural Gas & Other
     
Public Utilities, Inventory [Line Items]      
Weighted average cost inventory amount $ 0.2 $ 0.1 $ 0.1