Company's Liability for Environmental Obligations (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Environmental Exit Cost [Line Items]
Total Balance at Beginning of Period	$ 2.7	$ 2.0
Additions	0.2	0.9
Less: Payments / Reductions	0.8	0.2
Total Balance at End of Period	2.1	2.7
Less: Current Portion	0.3	0.6
Noncurrent Balance at End of Period	1.8	2.1
Fitchburg Gas and Electric Light Company
Environmental Exit Cost [Line Items]
Total Balance at Beginning of Period	0.0
Additions	0.1
Less: Payments / Reductions	0.0
Total Balance at End of Period	0.1	0.0
Less: Current Portion	0.1
Noncurrent Balance at End of Period	0.0
Northern Utilities Inc
Environmental Exit Cost [Line Items]
Total Balance at Beginning of Period	2.7	2.0
Additions	0.1	0.9
Less: Payments / Reductions	0.8	0.2
Total Balance at End of Period	2.0	2.7
Less: Current Portion	0.2	0.6
Noncurrent Balance at End of Period	$ 1.8	$ 2.1