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Income Taxes - Additional Information (Detail) - USD ($) $ in Millions | 1 Months Ended | 6 Months Ended | 12 Months Ended |
--- | --- | --- | --- | ---
Dec. 31, 2017 | Jun. 30, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Jun. 30, 2017
Income Taxes [Line Items] |  |  |  |  |
Regulatory Liability | $ 58.1 | $ 61.3 | $ 58.1 |  | $ 16.6
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 | $ 47.1 | 48.9 |  |
Tax Year 2018 |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |
Corporate federal income tax | 21.00% | 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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