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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