Company's Liability for Environmental Obligations (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2016
Environmental Exit Cost [Line Items]
Total Balance at Beginning of Period	$ 1.9	$ 2.8
Additions	0.4	1.8
Less: Payments / Reductions	0.2	2.7
Total Balance at End of Period	2.1	1.9
Less: Current Portion			$ 0.5	$ 0.4
Noncurrent Balance			1.6	1.5
Total Balance at End of period	1.9	2.8	2.1	1.9
Fitchburg Gas and Electric Light Company
Environmental Exit Cost [Line Items]
Total Balance at Beginning of Period	0.1	1.2
Less: Payments / Reductions		1.1
Total Balance at End of Period	0.1	0.1
Less: Current Portion				0.1
Noncurrent Balance			0.1
Total Balance at End of period	0.1	1.2	0.1	0.1
Northern Utilities Inc
Environmental Exit Cost [Line Items]
Total Balance at Beginning of Period	1.8	1.6
Additions	0.4	1.8
Less: Payments / Reductions	0.2	1.6
Total Balance at End of Period	2.0	1.8
Less: Current Portion			0.5	0.3
Noncurrent Balance			1.5	1.5
Total Balance at End of period	$ 1.8	$ 1.6	$ 2.0	$ 1.8