Components of Exchange Gas Receivable (Detail) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Receivables [Line Items]
Total Exchange Gas Receivable	$ 7.4	$ 4.9
Northern Utilities Inc
Receivables [Line Items]
Total Exchange Gas Receivable	6.7	4.4
Fitchburg
Receivables [Line Items]
Total Exchange Gas Receivable	$ 0.7	$ 0.5