Components of Exchange Gas Receivable (Detail) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Receivables [Line Items]
Total Exchange Gas Receivable	$ 0.2	$ 5.8	$ 2.1
Northern Utilities Inc
Receivables [Line Items]
Total Exchange Gas Receivable		5.4	1.9
Fitchburg Gas and Electric Light Company
Receivables [Line Items]
Total Exchange Gas Receivable	$ 0.2	$ 0.4	$ 0.2