Company's Liability for Environmental Obligations (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2016	Dec. 31, 2015
Environmental Exit Cost [Line Items]
Total Balance at Beginning of period	$ 2.8	$ 5.5
Additions	1.8	2.9
Less: Payments / Reductions	2.7	5.6
Total Balance at End of period	1.9	2.8
Less: Current Portion			$ 0.4	$ 1.3
Noncurrent Balance			1.5	1.5
Total Balance at End of period	2.8	5.5	1.9	2.8
Fitchburg Gas and Electric Light Company
Environmental Exit Cost [Line Items]
Total Balance at Beginning of period	1.2	1.9
Less: Payments / Reductions	1.1	0.7
Total Balance at End of period	0.1	1.2
Less: Current Portion			0.1	0.2
Noncurrent Balance				1.0
Total Balance at End of period	1.2	1.9	0.1	1.2
Northern Utilities Inc
Environmental Exit Cost [Line Items]
Total Balance at Beginning of period	1.6	3.6
Additions	1.8	2.9
Less: Payments / Reductions	1.6	4.9
Total Balance at End of period	1.8	1.6
Less: Current Portion			0.3	1.1
Noncurrent Balance			1.5	0.5
Total Balance at End of period	$ 1.6	$ 3.6	$ 1.8	$ 1.6