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Components of Exchange Gas Receivable (Detail) - USD ($)
$ in Millions
Sep. 30, 2018
Dec. 31, 2017
Sep. 30, 2017
Receivables [Line Items]      
Total Exchange Gas Receivable $ 10.1 $ 5.8 $ 9.5
Northern Utilities Inc      
Receivables [Line Items]      
Total Exchange Gas Receivable 9.5 5.4 8.9
Fitchburg Gas and Electric Light Company      
Receivables [Line Items]      
Total Exchange Gas Receivable $ 0.6 $ 0.4 $ 0.6