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Income Taxes - Additional Information (Detail) - USD ($) $ in Millions | 1 Months Ended | 3 Months Ended | 12 Months Ended |
--- | --- | --- | --- | ---
Dec. 31, 2017 | Mar. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Mar. 31, 2017
Income Taxes [Line Items] |  |  |  |  |
Corporate federal income tax |  | 21.00% |  |  |
Regulatory Liability | $ 58.1 | $ 60.0 | $ 58.1 |  | $ 13.5
Regulatory liability, expected flow back to customers |  | $ 47.1 |  |  |
Regulatory liability, expected pass back to ratepayers |  |  | 1.8 |  |
Tax benefit for New Hampshire business enterprise tax credits |  |  |  | $ 0.7 |
Deferred tax assets, operating loss carryforwards, federal | 1.1 |  | 1.1 |  |
Net Operating Loss Carryforwards | 12.7 |  | $ 12.7 |  |
NOL carryforward assets expiration date |  |  | 2029 |  |
Alternative minimum tax credit carryforwards | 3.5 |  | $ 3.5 |  |
Minimum |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |
Average Rate Assumption Method estimated flow back period |  | 15 years |  |  |
Maximum |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |
Average Rate Assumption Method estimated flow back period |  | 20 years |  |  |
Income Tax Related Liabilities |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |
Regulatory Liability | $ 48.9 | $ 49.1 | 48.9 |  |
Tax Year 2018 |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |
Corporate federal income tax | 21.00% |  |  |  |
Deferred tax assets, operating loss carryforwards, federal | $ 10.1 |  | $ 10.1 |  |
Federal, Maine, Massachusetts, and New Hampshire Tax Authorities |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |
Tax examination description |  | The Company remains subject to examination by Federal, Maine, Massachusetts, and New Hampshire tax authorities for the tax periods ended December 31, 2014; December 31, 2015; and December 31, 2016 |  |  |

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- Definition Amount before allocation of valuation allowances of deferred tax asset attributable to deductible domestic operating loss carryforwards. Excludes state and local operating loss carryforwards. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32621-109319 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 25 -Paragraph 20 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=96869850&loc=d3e28680-109314 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32632-109319 + Details
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- Definition Amount before allocation of valuation allowances of deferred tax asset attributable to deductible alternative minimum tax credit carryforwards. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32559-109319 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32632-109319 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32621-109319 + Details
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- Definition Amount of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to tax credits. Including, but not limited to, research credit, foreign tax credit, investment tax credit, and other tax credits. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 12 -URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32687-109319 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 13 -URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32698-109319 + Details
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