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Components of Exchange Gas Receivable (Detail) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Receivables [Line Items]    
Total Exchange Gas Receivable $ 5.8 $ 8.3
Northern Utilities Inc    
Receivables [Line Items]    
Total Exchange Gas Receivable 5.4 7.8
Fitchburg Gas and Electric Light Company    
Receivables [Line Items]    
Total Exchange Gas Receivable $ 0.4 $ 0.5