Components of Exchange Gas Receivable (Detail) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023
Receivables [Line Items]
Total Exchange Gas Receivable	$ 2.2	$ 9.4	$ 4.7
Northern Utilities Inc
Receivables [Line Items]
Total Exchange Gas Receivable	1.9	8.6	4.0
Fitchburg
Receivables [Line Items]
Total Exchange Gas Receivable	$ 0.3	$ 0.8	$ 0.7