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Components of Exchange Gas Receivable (Detail) - USD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Receivables [Line Items]    
Total Exchange Gas Receivable $ 4.9 $ 6.1
Northern Utilities Inc    
Receivables [Line Items]    
Total Exchange Gas Receivable 4.4 5.5
Fitchburg    
Receivables [Line Items]    
Total Exchange Gas Receivable $ 0.5 $ 0.6