v3.25.2
Summary of Significant Accounting Policies - Additional Information (Detail)
$ in Thousands
 6 Months Ended 12 Months Ended
Jan. 31, 2025
USD ($)
Jun. 30, 2025
USD ($)
Subsidiary
mi
Dec. 31, 2024
USD ($)
Megawatts
Jul. 08, 2025
USD ($)
Mar. 31, 2025
USD ($)
Jun. 30, 2024
USD ($)
Significant Accounting Policies [Line Items]            
Number of Subsidiaries | Entity | Subsidiary   3        
Length of Pipeline | mi   85        
Cost of removal obligation   $ 147,400 $ 139,200     $ 134,900
Investments in trading securities   6,100 $ 6,300     5,800
Goodwill   1,600        
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        
Asset Acquisition Consideration Transferred   100,000        
Net Utility Plant   1,644,400 $ 1,539,600     1,454,800
Allowance for doubtful accounts   1,900 2,400     2,200
Regulatory assets   99,200 112,000     114,100
Regulatory Liabilities   73,500 64,000     57,800
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 $ 71,400          
Other Deferred Charges [Member]            
Significant Accounting Policies [Line Items]            
Regulatory assets   14,700 14,100     13,100
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   $ 1,900 $ 2,300     2,200
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   $ 8,300 $ 7,900     7,200
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        
Subsequent Event [Member] | Bangor Natural Gas            
Significant Accounting Policies [Line Items]            
Debt Instrument, Face Amount       $ 32,000    
Environmental and Rate Case Costs and Other Expenditures | Recovered over the next seven years            
Significant Accounting Policies [Line Items]            
Regulatory assets   7,800        
ISO-NE Obligations            
Significant Accounting Policies [Line Items]            
Cash Deposits   $ 6,000 5,000     2,300
Maximum            
Significant Accounting Policies [Line Items]            
Lease term   12 months        
Deferred Compensation Plan [Member]            
Significant Accounting Policies [Line Items]            
Investments in trading securities   $ 2,900 $ 2,200     $ 1,900