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Other long-term liabilities - Narrative (Details) - USD ($)
$ in Thousands
 1 Months Ended 12 Months Ended
Jun. 30, 2021
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Debt Instrument [Line Items]        
Transfers from advances in aid of construction to contributions in aid of construction   $ 672 $ 238  
Undiscounted, unescalated cost of environmental cleanup activities   48,976 46,187  
Environmental remediation obligation   41,714 40,772 $ 42,457
Accrual for environmental loss contingencies to be incurred over next three years   $ 25,038    
Remaining period to incur cash flows for environmental cleanup   29 years    
Regulatory assets   $ 1,321,059 1,327,683  
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      
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% 5.37%    
Environmental costs        
Debt Instrument [Line Items]        
Environmental remediation, rate recovery period   7 years    
Regulatory assets   $ 62,340 $ 66,779  
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)   4.70%