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v3.25.1

Summary of Significant Accounting Policies - Additional Information (Detail) $ in Thousands |  | 3 Months Ended | 12 Months Ended
--- | --- | --- | ---
Jan. 31, 2025 USD ($) | Mar. 31, 2025 USD ($) | Mar. 31, 2024 USD ($) Subsidiary mi | Dec. 31, 2024 USD ($) Megawatts
Significant Accounting Policies [Line Items] |  |  |  |
Number of Subsidiaries | Entity | Subsidiary |  |  | 3 |
Length of Pipeline | mi |  |  | 85 |
Cost of removal obligation |  | $ 143,900 | $ 130,600 | $ 139,200
Investments in trading securities |  | 6,100 | 5,900 | $ 6,300
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 |  |
Asset Acquisition Consideration Transferred |  | 100,000 |  |
Net Utility Plant |  | 1,618,900 | 1,429,800 | $ 1,539,600
Allowance for doubtful accounts |  | 2,900 | 2,800 | 2,400
Regulatory assets |  | 100,900 | 117,200 | 112,000
Regulatory Liabilities |  | 76,500 | 51,900 | 64,000
Cash Paid for Acquisition |  | 86,000 |  |
Loan commitment to fund an acquisition |  | 86,000 |  |
Electric And Gas Division [Member] |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |
Business Combination Price Of Acquisition Expected | $ 71,200 |  |  |
Net Utility Plant |  | 66,700 |  |
Working Capital |  | 5,400 |  |
Regulatory Liabilities |  | 900 |  |
Other Deferred Charges [Member] |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |
Regulatory assets |  | 14,300 | 12,600 | 14,100
Unbilled Revenues [member] |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |
Allowance for doubtful accounts |  | 100 | 100 | 100
Accounts Receivable [member] |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |
Allowance for doubtful accounts |  | 2,800 | $ 2,700 | $ 2,300
Utilities |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |
Number of Subsidiaries | Entity | Subsidiary |  |  | 3 |
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 |  | 8,200 | $ 6,900 | $ 7,900
Environmental and Rate Case Costs and Other Expenditures | Recovered over the next seven years |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |
Regulatory assets |  | 7,300 |  |
ISO-NE Obligations |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |
Cash Deposits |  | $ 6,600 | 3,100 | 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 |  | $ 2,500 | $ 1,800 | $ 2,200

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