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Components of Exchange Gas Receivable (Detail) - USD ($)
$ in Millions
Dec. 31, 2023
Dec. 31, 2022
Receivables [Line Items]    
Total Exchange Gas Receivable $ 9.4 $ 18.0
Northern Utilities Inc    
Receivables [Line Items]    
Total Exchange Gas Receivable 8.6 16.3
Fitchburg    
Receivables [Line Items]    
Total Exchange Gas Receivable $ 0.8 $ 1.7