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Commitments and Contingencies (Tables) | 12 Months Ended
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Dec. 31, 2015
Environmental Obligations | The following table sets forth a summary of changes in the Company’s liability for Environmental Obligations for the years ended December 31, 2015 and 2014. Environmental Obligations
|  | (millions) |
|  | Fitchburg |  |  | Northern Utilities |  |  | Total |
|  | 2015 |  |  | 2014 |  |  | 2015 |  |  | 2014 |  |  | 2015 |  |  | 2014 |
Total Balance at Beginning of Period |  | $ | 1.9 |  |  | $ | 12.0 |  |  | $ | 3.6 |  |  | $ | 2.8 |  |  | $ | 5.5 |  |  | $ | 14.8 |
Additions |  |  | — |  |  |  | — |  |  |  | 2.9 |  |  |  | 1.3 |  |  |  | 2.9 |  |  |  | 1.3 |
Less: Payments / Reductions |  |  | 0.7 |  |  |  | 10.1 |  |  |  | 4.9 |  |  |  | 0.5 |  |  |  | 5.6 |  |  |  | 10.6 |
Total Balance at End of Period |  | $ | 1.2 |  |  | $ | 1.9 |  |  | $ | 1.6 |  |  | $ | 3.6 |  |  | $ | 2.8 |  |  | $ | 5.5 |
Less: Current Portion |  |  | 0.2 |  |  |  | 1.9 |  |  |  | 1.1 |  |  |  | 1.6 |  |  |  | 1.3 |  |  |  | 3.5 |
Noncurrent Balance at December 31, 2014 |  | $ | 1.0 |  |  | $ | — |  |  | $ | 0.5 |  |  | $ | 2.0 |  |  | $ | 1.5 |  |  | $ | 2.0 |

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- Definition Schedule Of Environmental Remediation Obligations Table [Text Block] + References No definition available. + Details
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