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Regulatory matters - Regulatory Assets and Liabilities (Detail)
$ in Thousands
 12 Months Ended
Dec. 31, 2021
USD ($)
Dec. 31, 2020
USD ($)
Mar. 01, 2020
MWac
Regulatory Liabilities [Line Items]      
Total regulatory assets $ 1,167,625 $ 846,519  
Less: current regulatory assets (158,212) (64,090)  
Non-current regulatory assets 1,009,413 782,429  
Total regulatory liabilities 576,189 601,518  
Less: current regulatory liabilities (65,809) (38,483)  
Non-current regulatory liabilities $ 510,380 563,035  
Capital expenditure shortfall refundable to customers (percent) 80.00%    
Income taxes      
Regulatory Liabilities [Line Items]      
Total regulatory liabilities $ 295,720 322,317  
Cost of removal      
Regulatory Liabilities [Line Items]      
Total regulatory liabilities 191,981 200,739  
Pension and Postremployment benefits      
Regulatory Liabilities [Line Items]      
Total regulatory liabilities 34,468 26,311  
Fuel and commodity cost adjustments      
Regulatory Liabilities [Line Items]      
Total regulatory liabilities 18,229 20,136  
Clean energy and other customer programs      
Regulatory Liabilities [Line Items]      
Total regulatory liabilities 14,829 10,440  
Rate adjustment mechanism      
Regulatory Liabilities [Line Items]      
Total regulatory liabilities 3,316 5,214  
Other      
Regulatory Liabilities [Line Items]      
Total regulatory liabilities 17,646 16,361  
Fuel and commodity cost adjustments      
Regulatory Liabilities [Line Items]      
Total regulatory assets 339,900 18,094  
Retired generating plant      
Regulatory Liabilities [Line Items]      
Total regulatory assets 185,073 194,192  
Coal generation capacity (MW) | MWac     200
Pension and Postremployment benefits      
Regulatory Liabilities [Line Items]      
Total regulatory assets $ 134,141 178,403  
Future service years of employees 10 years    
Rate adjustment mechanism      
Regulatory Liabilities [Line Items]      
Total regulatory assets $ 117,309 99,853  
Retroactive rate adjustment collection period 24 months    
Regulatory asset, amortization period 26 years    
Environmental remediation      
Regulatory Liabilities [Line Items]      
Total regulatory assets $ 81,802 87,308  
Environmental remediation, rate recovery period 7 years    
Income taxes      
Regulatory Liabilities [Line Items]      
Total regulatory assets $ 79,472 77,730  
Deferred capitalized costs      
Regulatory Liabilities [Line Items]      
Total regulatory assets $ 62,599 34,398  
Regulatory asset, amortization period 20 years    
Capitalized operating and maintenance costs, recovery rate, (percent) 2.43%    
Capitalized operating and maintenance costs, recovery period 29 years    
Wildfire mitigation and vegetation management      
Regulatory Liabilities [Line Items]      
Total regulatory assets $ 35,789 22,736  
Debt premium      
Regulatory Liabilities [Line Items]      
Total regulatory assets 34,204 35,688  
Asset retirement obligation      
Regulatory Liabilities [Line Items]      
Total regulatory assets 26,810 26,546  
Clean energy and other customer programs      
Regulatory Liabilities [Line Items]      
Total regulatory assets 26,015 26,400  
Rate review costs      
Regulatory Liabilities [Line Items]      
Total regulatory assets 9,167 8,054  
Long-term maintenance contract      
Regulatory Liabilities [Line Items]      
Total regulatory assets 9,134 14,405  
Other      
Regulatory Liabilities [Line Items]      
Total regulatory assets $ 26,210 $ 22,712