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Other long-term liabilities - Additional Information (Detail)
$ in Thousands
 1 Months Ended 12 Months Ended
Jun. 30, 2021
USD ($)
shares
Jan. 31, 2021
USD ($)
Dec. 31, 2022
USD ($)
shares
Dec. 31, 2021
USD ($)
Dec. 31, 2022
$ / shares
Dec. 31, 2020
USD ($)
Debt Instrument [Line Items]            
Green Equity Units issued (in shares) | 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   $ 222,378      
Contract adjustment payments, accretion period 3 years          
Transfers from advances in aid of construction to contributions in aid of construction     1,299 $ 6,376    
Undiscounted, unescalated cost of environmental cleanup activities     48,346 57,167    
Environmental remediation obligation     42,457 55,224   $ 69,383
Accrual for environmental loss contingencies to be incurred over next four years     27,410      
Regulatory assets     1,271,501 1,167,625    
Note payable to related party     $ 25,808 25,808   30,493
Maverick Creek and Sugar Creek Wind Facilities            
Debt Instrument [Line Items]            
Total purchase price   $ 18,641        
Equity Method Investee            
Debt Instrument [Line Items]            
Note payable to related party       $ 25,808   $ 30,493
Note payable to related party            
Debt Instrument [Line Items]            
Interest rate (percent)       4.00%   0.675%
Preferred shares, Series C            
Debt Instrument [Line Items]            
Redeemable preferred stock issued (in shares) | shares     100      
Preferred stock redemption price per share (in CAD per share) | $ / shares         $ 53,400  
Environmental costs            
Debt Instrument [Line Items]            
Environmental remediation, rate recovery period     7 years      
Regulatory assets     $ 70,529 $ 81,802    
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)     4.20%      
U.S. Dollar 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%