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v3.25.3

Summary of Significant Accounting Policies - Additional Information (Detail) $ in Thousands |  |  | 1 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | --- | --- | ---
Mar. 31, 2025 USD ($) | Jan. 31, 2025 USD ($) | Dec. 31, 2024 USD ($) Megawatts | Sep. 30, 2025 USD ($) Subsidiary mi | Sep. 30, 2024 USD ($) | Dec. 31, 2024 USD ($) Megawatts
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Number of Subsidiaries | Entity | Subsidiary |  |  |  | 3 |  |
Length of Pipeline | mi |  |  |  | 85 |  |
Cost of removal obligation |  |  | $ 139,200 | $ 151,400 | $ 139,300 | $ 139,200
Investments in trading securities |  |  | 6,300 | 6,100 | 5,800 | 6,300
Goodwill |  |  | 0 | 1,600 | 0 | $ 0
Deductible portion of goodwill |  |  |  | $ 400 |  |
Amortization period |  |  |  | 15 years |  |
Long Term Renewable Contracts |  |  |  |  |  | 30.00%
Business Acquisition Liabilities Assumed |  |  |  | $ 30,000 |  |
Energy Storage Systems 1 | Megawatts |  |  |  |  |  | 5,000
Mid Duration Storage | Megawatts |  |  |  |  |  | 1,500
Cash And Cash Equivalents Acquired |  |  |  | 70,000 |  |
Acquisition, Net of Cash Acquired |  |  |  | 71,400 | 0 |
Asset Acquisition Consideration Transferred |  |  |  | 100,000 |  |
Net Utility Plant |  |  | 1,539,600 | 1,682,900 | 1,502,400 | $ 1,539,600
Allowance for doubtful accounts |  |  | 2,400 | 1,200 | 2,000 | 2,400
Regulatory assets |  |  | 112,000 | 105,600 | 110,800 | 112,000
Regulatory Liabilities |  |  | 64,000 | 70,600 | 68,700 | 64,000
Electric And Gas Division [Member] |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Business Combination Price Of Acquisition Expected |  | $ 71,400 |  |  |  |
Other Deferred Charges [Member] |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Regulatory assets |  |  | 14,100 | 12,500 | 12,400 | 14,100
Unbilled Revenues [member] |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Allowance for doubtful accounts |  |  | 100 | 100 | 100 | 100
Accounts Receivable [member] |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Allowance for doubtful accounts |  |  | $ 2,300 | $ 1,200 | 1,900 | 2,300
Utilities |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Number of Subsidiaries | Entity | Subsidiary |  |  |  | 4 |  |
Avangrid Enterprises Inc |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Acquisition, Net of Cash Acquired | $ 86,000 |  |  |  |  |
Fitchburg Gas And Electric Light Company [Member] |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Energy Storage Systems 1 | Megawatts |  |  | 5,000 |  |  |
Mid Duration Storage | Megawatts |  |  | 1,500 |  |  |
Fitchburg Gas And Electric Light Company [Member] | Other Deferred Charges [Member] | Electric And Gas Division [Member] |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Hardship accounts in regulatory assets |  |  | $ 7,900 | $ 8,000 | 6,800 | 7,900
Bangor Natural Gas |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Goodwill |  |  |  | 1,600 |  |
Net Utility Plant |  |  |  | 66,700 |  |
Working Capital |  |  |  | 3,800 |  |
Noncurrent assets |  |  |  | 200 |  |
Regulatory Liabilities |  |  |  | 900 |  |
Environmental and Rate Case Costs and Other Expenditures | Recovered over the next seven years |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Regulatory assets |  |  |  | 6,700 |  |
ISO-NE Obligations |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Cash Deposits |  |  | 5,000 | $ 11,800 | 5,800 | 5,000
Maximum |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Lease term |  |  |  | 12 months |  |
Deferred Compensation Plan [Member] |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Investments in trading securities |  |  | $ 2,200 | $ 3,200 | $ 2,100 | $ 2,200

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- Definition Hardship accounts in regulatory assets. + References No definition available. + Details
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- Definition Length of natural gas transmission pipeline. + References No definition available. + Details
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- Definition Long-Term Renewable Contracts + References No definition available. + Details
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- Definition Mid-Duration Storage + References No definition available. + Details
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- Definition Number of Subsidiaries. + References No definition available. + Details
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- Definition Significant Accounting Policies [Line Items] + References No definition available. + Details
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- Definition Working Capital + References No definition available. + Details
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