Company's Liability for Environmental Obligations (Detail) - USD ($)
$ in Millions |
6 Months Ended |
|
|
|
Jun. 30, 2016 |
Jun. 30, 2015 |
Jun. 30, 2016 |
Dec. 31, 2015 |
Jun. 30, 2015 |
| Environmental Exit Cost [Line Items] |
|
|
|
|
|
| Total Balance at Beginning of period |
$ 2.8
|
$ 5.5
|
|
|
|
| Additions |
0.9
|
0.3
|
|
|
|
| Less: Payments / Reductions |
0.7
|
0.4
|
|
|
|
| Total Balance at End of period |
3.0
|
5.4
|
|
|
|
| Less: Current Portion |
|
|
$ 0.3
|
$ 1.3
|
$ 3.4
|
| Noncurrent Balance |
|
|
2.7
|
1.5
|
2.0
|
| Total Balance at End of period |
3.0
|
5.4
|
3.0
|
2.8
|
5.4
|
| Fitchburg Gas and Electric Light Company |
|
|
|
|
|
| Environmental Exit Cost [Line Items] |
|
|
|
|
|
| Total Balance at Beginning of period |
1.2
|
1.9
|
|
|
|
| Additions |
|
0.1
|
|
|
|
| Less: Payments / Reductions |
0.1
|
0.2
|
|
|
|
| Total Balance at End of period |
1.1
|
1.8
|
|
|
|
| Less: Current Portion |
|
|
0.1
|
|
1.8
|
| Noncurrent Balance |
|
|
1.0
|
|
|
| Total Balance at End of period |
1.1
|
1.8
|
1.1
|
1.2
|
1.8
|
| Northern Utilities Inc |
|
|
|
|
|
| Environmental Exit Cost [Line Items] |
|
|
|
|
|
| Total Balance at Beginning of period |
1.6
|
3.6
|
|
|
|
| Additions |
0.9
|
0.2
|
|
|
|
| Less: Payments / Reductions |
0.6
|
0.2
|
|
|
|
| Total Balance at End of period |
1.9
|
3.6
|
|
|
|
| Less: Current Portion |
|
|
0.2
|
|
1.6
|
| Noncurrent Balance |
|
|
1.7
|
|
2.0
|
| Total Balance at End of period |
$ 1.9
|
$ 3.6
|
$ 1.9
|
$ 1.6
|
$ 3.6
|