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Income Taxes - Additional Information (Detail) - USD ($) | 1 Months Ended | 3 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | ---
Dec. 31, 2017 | Mar. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017 | Sep. 30, 2018 | Mar. 31, 2018
Income Taxes [Line Items] |  |  |  |  |  |
Regulatory Liability |  | $ 62,400,000 | $ 58,500,000 |  |  | $ 60,000,000
Regulatory liability, expected flow back to customers |  | $ 47,100,000 |  | $ 48,900,000 |  |
Tax provision |  |  | 2,400,000 |  |  |
Net Operating Loss Carryforwards |  |  | 10,800,000 |  | $ 3,700,000 |
Calculated federal current tax |  |  | 7,700,000 |  |  |
Deferred tax assets, operating loss carryforwards, federal |  |  | 7,700,000 |  |  |
Federal current taxes payable |  |  | $ 0 |  |  |
NOL carryforward assets expiration date |  |  | 2029 |  |  |
Alternative minimum tax credit carryforwards | $ 3,500,000 |  |  | 3,500,000 |  |
Reconciling Mechanisms |  |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |  |
Regulatory liability, expected pass back to ratepayers |  |  |  | 1,800,000 |  |
Net Operating Loss Carryforward Assets |  |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |  |
Regulatory liability, expected pass back to ratepayers |  |  | $ 3,400,000 | 5,800,000 |  |
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 |  |  |  |
Fitchburg Gas and Electric Light Company |  |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |  |
Average Rate Assumption Method estimated flow back period |  | 15 years |  |  |  |
Income Tax Related Liabilities |  |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |  |
Regulatory Liability | $ 48,900,000 | $ 48,200,000 | $ 47,000,000 | $ 48,900,000 |  | $ 49,100,000
Electric Ratepayers | Fitchburg Gas and Electric Light Company |  |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |  |
Regulatory liability, expected flow back to customers |  | 10,100,000 |  |  |  |
Gas Ratepayers | Fitchburg Gas and Electric Light Company |  |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |  |
Regulatory liability, expected flow back to customers |  | $ 10,400,000 |  |  |  |
Tax Year 2018 |  |  |  |  |  |
Income Taxes [Line Items] |  |  |  |  |  |
Corporate federal income tax | 21.00% | 21.00% |  |  |  |
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, 2015; December 31, 2016; and December 31, 2017. |  |  |  |

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