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Regulatory matters - Regulatory Assets and Liabilities (Detail)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
USD ($)
Mar. 01, 2020
MWac
Dec. 31, 2019
USD ($)
Regulatory Liabilities [Line Items]      
Total regulatory assets $ 845,471   $ 559,887
Less: current regulatory assets (63,042)   (50,213)
Non-current regulatory assets 782,429   509,674
Total regulatory liabilities 601,518   607,378
Less: current regulatory liabilities (38,483)   (41,683)
Non-current regulatory liabilities $ 563,035   565,695
Capital expenditure shortfall refundable to customers (percent) 80.00%    
Income taxes      
Regulatory Liabilities [Line Items]      
Total regulatory liabilities $ 322,317   321,960
Cost of removal      
Regulatory Liabilities [Line Items]      
Total regulatory liabilities 200,739   205,739
Pension and Postremployment benefits      
Regulatory Liabilities [Line Items]      
Total regulatory liabilities 26,311   22,256
Fuel and commodity cost adjustments      
Regulatory Liabilities [Line Items]      
Total regulatory liabilities 20,136   17,729
Rate adjustment mechanism      
Regulatory Liabilities [Line Items]      
Total regulatory liabilities 5,214   10,446
Clean energy and other customer programs      
Regulatory Liabilities [Line Items]      
Total regulatory liabilities 10,440   6,871
Rate base offset      
Regulatory Liabilities [Line Items]      
Total regulatory liabilities $ 6,874   8,719
Rate base offset | Minimum      
Regulatory Liabilities [Line Items]      
Regulatory liability, amortization period 10 years    
Rate base offset | Maximum      
Regulatory Liabilities [Line Items]      
Regulatory liability, amortization period 16 years    
Other      
Regulatory Liabilities [Line Items]      
Total regulatory liabilities $ 9,487   13,658
Retired generating plant      
Regulatory Liabilities [Line Items]      
Total regulatory assets 194,192   0
Coal generation capacity (MW) | MWac   200  
Pension and Postremployment benefits      
Regulatory Liabilities [Line Items]      
Total regulatory assets $ 178,403   143,292
Future service years of employees 10 years    
Rate adjustment mechanism      
Regulatory Liabilities [Line Items]      
Total regulatory assets $ 99,853   69,121
Retroactive rate adjustment collection period 24 months    
Regulatory asset, amortization period 26 years    
Environmental remediation      
Regulatory Liabilities [Line Items]      
Total regulatory assets $ 87,308   82,300
Environmental remediation, rate recovery period 7 years    
Income taxes      
Regulatory Liabilities [Line Items]      
Total regulatory assets $ 77,730   71,506
Debt premium      
Regulatory Liabilities [Line Items]      
Total regulatory assets 35,688   42,150
Fuel and commodity cost adjustments      
Regulatory Liabilities [Line Items]      
Total regulatory assets 18,094   23,433
Clean energy and other customer programs      
Regulatory Liabilities [Line Items]      
Total regulatory assets 26,400   25,859
Deferred capitalized costs      
Regulatory Liabilities [Line Items]      
Total regulatory assets $ 34,398   38,833
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    
Asset retirement obligation      
Regulatory Liabilities [Line Items]      
Total regulatory assets $ 26,546   23,841
Wildfire mitigation and vegetation management      
Regulatory Liabilities [Line Items]      
Total regulatory assets 22,736   5,043
Long-term maintenance contract      
Regulatory Liabilities [Line Items]      
Total regulatory assets 14,405   13,264
Rate review costs      
Regulatory Liabilities [Line Items]      
Total regulatory assets 8,054   7,205
Other      
Regulatory Liabilities [Line Items]      
Total regulatory assets $ 21,664   $ 14,040