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Other long-term liabilities - Additional Information (Detail) - USD ($)
$ in Thousands
 1 Months Ended 12 Months Ended
Jun. 30, 2021
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Debt Instrument [Line Items]        
Green Equity Units issued (in shares) 23,000,000      
Green Equity Units, annual distributions (percent) 7.75%      
Green Equity Units, share purchase contract, interest rate (percent) 6.57%      
Contract adjustment payments, excluding interest $ 222,378      
Contract adjustment payments, accretion period 3 years      
Transfers from advances in aid of construction to contributions in aid of construction   $ 238 $ 1,299  
Undiscounted, unescalated cost of environmental cleanup activities   46,187 48,346  
Environmental remediation obligation   40,772 42,457 $ 55,224
Accrual for environmental loss contingencies to be incurred over next three years   $ 25,713    
Remaining period to incur cash flows for environmental cleanup   27 years    
Regulatory assets   $ 1,327,683 1,271,501  
Equity Method Investee        
Debt Instrument [Line Items]        
Note payable to related party       $ 25,808
Related Party        
Debt Instrument [Line Items]        
Note payable to related party   $ 25,808 25,808  
Note payable to related party        
Debt Instrument [Line Items]        
Interest rate (percent)       4.00%
Preferred shares, Series C | St. Leon Wind Energy LP        
Debt Instrument [Line Items]        
Redeemable preferred stock issued (in shares)   100    
Debt instrument, redeemed amount   $ 14,515    
Gain (loss) on settlement   $ 2,377    
Environmental costs        
Debt Instrument [Line Items]        
Environmental remediation, rate recovery period   7 years    
Regulatory assets   $ 66,779 $ 70,529  
Minimum        
Debt Instrument [Line Items]        
Other liability repayment period   5 years    
Accrual for environmental cleanup, discount rate (percent)   3.40%    
Maximum        
Debt Instrument [Line Items]        
Other liability repayment period   40 years    
Accrual for environmental cleanup, discount rate (percent)   4.30%    
Senior unsecured notes (Green Equity Units) | Senior Unsecured Notes        
Debt Instrument [Line Items]        
Proceeds from green equity units $ 1,150,000      
Weighted average coupon 1.18% 1.18%