Company's Liability for Environmental Obligations (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Environmental Exit Cost [Line Items]
Total Balance at Beginning of Period	$ 2.1	$ 1.9
Additions	0.6	0.5
Less: Payments / Reductions	0.6	0.2
Total Balance at End of Period	2.1	2.2
Less: Current Portion			$ 0.6	$ 0.5
Noncurrent Balance			1.5	1.7
Total Balance at End of period	2.1	2.2	2.1	2.2
Fitchburg Gas and Electric Light Company
Environmental Exit Cost [Line Items]
Total Balance at Beginning of Period	0.1	0.1
Less: Payments / Reductions	0.1
Total Balance at End of Period		0.1
Less: Current Portion				0.1
Total Balance at End of period	0.1	0.1		0.1
Northern Utilities Inc
Environmental Exit Cost [Line Items]
Total Balance at Beginning of Period	2.0	1.8
Additions	0.6	0.5
Less: Payments / Reductions	0.5	0.2
Total Balance at End of Period	2.1	2.1
Less: Current Portion			0.6	0.4
Noncurrent Balance			1.5	1.7
Total Balance at End of period	$ 2.1	$ 2.1	$ 2.1	$ 2.1