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Commitments and Contingencies (Tables) | 12 Months Ended
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Dec. 31, 2017
Environmental Obligations Recognized by Company | The following table sets forth a summary of changes in the Company’s liability for Environmental Obligations for the years ended December 31, 2017 and 2016. Environmental Obligations
|  | (millions) |
|  | Fitchburg |  |  | Northern Utilities |  |  | Total |
|  | 2017 |  |  | 2016 |  |  | 2017 |  |  | 2016 |  |  | 2017 |  |  | 2016 |
Total Balance at Beginning of Period |  | $ | 0.1 |  |  | $ | 1.2 |  |  | $ | 1.8 |  |  | $ | 1.6 |  |  | $ | 1.9 |  |  | $ | 2.8 |
Additions |  |  | — |  |  |  | — |  |  |  | 0.4 |  |  |  | 1.8 |  |  |  | 0.4 |  |  |  | 1.8 |
Less: Payments / Reductions |  |  | — |  |  |  | 1.1 |  |  |  | 0.2 |  |  |  | 1.6 |  |  |  | 0.2 |  |  |  | 2.7 |
Total Balance at End of Period |  | $ | 0.1 |  |  | $ | 0.1 |  |  | $ | 2.0 |  |  | $ | 1.8 |  |  | $ | 2.1 |  |  | $ | 1.9 |
Less: Current Portion |  |  | — |  |  |  | 0.1 |  |  |  | 0.5 |  |  |  | 0.3 |  |  |  | 0.5 |  |  |  | 0.4 |
Noncurrent Balance at December 31, |  | $ | 0.1 |  |  | $ | — |  |  | $ | 1.5 |  |  | $ | 1.5 |  |  | $ | 1.6 |  |  | $ | 1.5 |

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- Definition Tabular disclosure of environmental loss contingencies by individual site. Does not include loss contingencies that are not environmental in nature. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 5.Y.Q2) -URI http://asc.fasb.org/extlink&oid=27011672&loc=d3e149879-122751 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 30 -Section 50 -Paragraph 10 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=6393242&loc=d3e13237-110859 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 30 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=6393242&loc=d3e13207-110859 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 30 -Section 50 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=6393242&loc=d3e13283-110859 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 30 -Section 50 -Paragraph 12 -URI http://asc.fasb.org/extlink&oid=6393242&loc=d3e13296-110859 + Details
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