Company's Liability for Environmental Obligations (Detail) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Dec. 31, 2016	Jun. 30, 2016
Environmental Exit Cost [Line Items]
Total Balance at Beginning of period	$ 2.0	$ 2.8
Additions	0.3	0.9
Less: Payments / Reductions	0.1	0.7
Total Balance at End of period	2.2	3.0
Less: Current Portion			$ 0.5	$ 0.4	$ 0.3
Noncurrent Balance			1.7	1.5	2.7
Total Balance at End of period	2.2	3.0	2.2	2.0	3.0
Fitchburg Gas and Electric Light Company
Environmental Exit Cost [Line Items]
Total Balance at Beginning of period	0.1	1.2
Less: Payments / Reductions		0.1
Total Balance at End of period	0.1	1.1
Less: Current Portion			0.1		0.1
Noncurrent Balance					1.0
Total Balance at End of period	0.1	1.1	0.1	0.1	1.1
Northern Utilities Inc
Environmental Exit Cost [Line Items]
Total Balance at Beginning of period	1.9	1.6
Additions	0.3	0.9
Less: Payments / Reductions	0.1	0.6
Total Balance at End of period	2.1	1.9
Less: Current Portion			0.4		0.2
Noncurrent Balance			1.7		1.7
Total Balance at End of period	$ 2.1	$ 1.9	$ 2.1	$ 1.9	$ 1.9