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Regulatory matters - Regulatory Assets and Liabilities (Detail) - USD ($)
$ in Thousands
Jun. 30, 2021
Dec. 31, 2020
Regulatory Liabilities [Line Items]    
Total regulatory assets $ 1,115,098 $ 845,471
Less: current regulatory assets (100,601) (63,042)
Non-current regulatory assets 1,014,497 782,429
Total regulatory liabilities 580,630 601,518
Less: current regulatory liabilities (34,256) (38,483)
Non-current regulatory liabilities 546,374 563,035
Income taxes    
Regulatory Liabilities [Line Items]    
Total regulatory liabilities 310,914 322,317
Cost of removal    
Regulatory Liabilities [Line Items]    
Total regulatory liabilities 197,616 200,739
Pension and Postremployment benefits    
Regulatory Liabilities [Line Items]    
Total regulatory liabilities 32,320 26,311
Fuel and commodity cost adjustments    
Regulatory Liabilities [Line Items]    
Total regulatory liabilities 5,086 20,136
Rate adjustment mechanism    
Regulatory Liabilities [Line Items]    
Total regulatory liabilities 1,598 5,214
Clean energy and other customer programs    
Regulatory Liabilities [Line Items]    
Total regulatory liabilities 14,658 10,440
Rate base offset    
Regulatory Liabilities [Line Items]    
Total regulatory liabilities 6,053 6,874
Other    
Regulatory Liabilities [Line Items]    
Total regulatory liabilities 12,385 9,487
Fuel and commodity cost adjustments    
Regulatory Liabilities [Line Items]    
Total regulatory assets 283,941 18,094
Retired generating plant    
Regulatory Liabilities [Line Items]    
Total regulatory assets 188,586 194,192
Pension and Postremployment benefits    
Regulatory Liabilities [Line Items]    
Total regulatory assets 172,501 178,403
Rate adjustment mechanism    
Regulatory Liabilities [Line Items]    
Total regulatory assets 102,500 99,853
Environmental remediation    
Regulatory Liabilities [Line Items]    
Total regulatory assets 86,058 87,308
Income taxes    
Regulatory Liabilities [Line Items]    
Total regulatory assets 80,778 77,730
Debt premium    
Regulatory Liabilities [Line Items]    
Total regulatory assets 33,277 35,688
Clean energy and other customer programs    
Regulatory Liabilities [Line Items]    
Total regulatory assets 26,047 26,400
Deferred capitalized costs    
Regulatory Liabilities [Line Items]    
Total regulatory assets 42,376 34,398
Asset retirement obligation    
Regulatory Liabilities [Line Items]    
Total regulatory assets 26,955 26,546
Wildfire mitigation and vegetation management    
Regulatory Liabilities [Line Items]    
Total regulatory assets 25,891 22,736
Long-term maintenance contract    
Regulatory Liabilities [Line Items]    
Total regulatory assets 11,920 14,405
Rate review costs    
Regulatory Liabilities [Line Items]    
Total regulatory assets 6,872 8,054
Other    
Regulatory Liabilities [Line Items]    
Total regulatory assets $ 27,396 $ 21,664