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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