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Regulatory matters - Regulatory Assets and Liabilities (Detail)
$ in Thousands
 1 Months Ended 12 Months Ended
Sep. 22, 2022
USD ($)
Dec. 31, 2022
USD ($)
Apr. 27, 2022
regulatory_case
Dec. 31, 2021
USD ($)
Mar. 01, 2020
MWac
Regulatory Liabilities [Line Items]          
Total regulatory assets   $ 1,271,501   $ 1,167,625  
Less: current regulatory assets   (190,393)   (158,212)  
Non-current regulatory assets   1,081,108   1,009,413  
Total regulatory liabilities   628,182   576,189  
Less: current regulatory liabilities   (69,865)   (65,809)  
Non-current regulatory liabilities   $ 558,317   510,380  
Capital expenditure shortfall refundable to customers (percent)   80.00%      
Retroactive rate adjustment collection period   24 months      
Regulatory asset, amortization period   26 years      
Income taxes          
Regulatory Liabilities [Line Items]          
Total regulatory liabilities   $ 312,671   295,720  
Cost of removal          
Regulatory Liabilities [Line Items]          
Total regulatory liabilities   191,173   191,981  
Pension and post-employment benefits          
Regulatory Liabilities [Line Items]          
Total regulatory liabilities   68,085   34,468  
Fuel and commodity costs adjustments          
Regulatory Liabilities [Line Items]          
Total regulatory liabilities   24,991   18,175  
Clean energy and other customer programs          
Regulatory Liabilities [Line Items]          
Total regulatory liabilities   11,572   14,829  
Rate adjustment mechanism          
Regulatory Liabilities [Line Items]          
Total regulatory liabilities   $ 343   3,316  
Rate adjustment mechanism | Minimum          
Regulatory Liabilities [Line Items]          
Regulatory assets, recovery period   1 year      
Rate adjustment mechanism | Maximum          
Regulatory Liabilities [Line Items]          
Regulatory assets, recovery period   5 years      
Other          
Regulatory Liabilities [Line Items]          
Total regulatory liabilities   $ 19,347   17,700  
Fuel and commodity costs adjustments          
Regulatory Liabilities [Line Items]          
Total regulatory assets   388,294   339,900  
Asset Retirement Obligation   21,283      
Number of regulatory cases | regulatory_case     2    
Qualified extraordinary costs $ 290,383        
Recovery denied (percent) 5.00%        
Fuel and commodity costs adjustments | Midwest Extreme Weather Event          
Regulatory Liabilities [Line Items]          
Issuance of securitized tariff bonds, authorized amount   221,646      
Fuel and commodity costs adjustments | Energy Transistion Costs          
Regulatory Liabilities [Line Items]          
Issuance of securitized tariff bonds, authorized amount   140,774      
Retired generating plant          
Regulatory Liabilities [Line Items]          
Total regulatory assets   174,609   185,073  
Coal generation capacity (MW) | MWac         200
Pension and post-employment benefits          
Regulatory Liabilities [Line Items]          
Total regulatory assets   $ 80,736   134,287  
Future service years of employees   10 years      
Rate adjustment mechanism          
Regulatory Liabilities [Line Items]          
Total regulatory assets   $ 136,198   117,309  
Environmental remediation          
Regulatory Liabilities [Line Items]          
Total regulatory assets   $ 70,529   81,802  
Environmental remediation, rate recovery period   7 years      
Income taxes          
Regulatory Liabilities [Line Items]          
Total regulatory assets   $ 97,414   79,472  
Deferred capitalized costs          
Regulatory Liabilities [Line Items]          
Total regulatory assets   $ 90,121   62,599  
Regulatory asset, amortization period   20 years      
Capitalized operating and maintenance costs, recovery rate, (percent)   2.43%      
Wildfire mitigation and vegetation management          
Regulatory Liabilities [Line Items]          
Total regulatory assets   $ 66,156   35,726  
Regulatory assets, recovery period   2 years      
Debt premium          
Regulatory Liabilities [Line Items]          
Total regulatory assets   $ 24,888   34,204  
Cost of removal          
Regulatory Liabilities [Line Items]          
Total regulatory assets   11,084   0  
Asset retirement obligation          
Regulatory Liabilities [Line Items]          
Total regulatory assets   27,172   26,810  
Clean energy and other customer programs          
Regulatory Liabilities [Line Items]          
Total regulatory assets   $ 28,145   25,857  
Clean energy and other customer programs | Minimum          
Regulatory Liabilities [Line Items]          
Regulatory assets, recovery period   6 years      
Clean energy and other customer programs | Maximum          
Regulatory Liabilities [Line Items]          
Regulatory assets, recovery period   10 years      
Rate review costs          
Regulatory Liabilities [Line Items]          
Total regulatory assets   $ 9,481   9,167  
Rate review costs | Minimum          
Regulatory Liabilities [Line Items]          
Costs capitalized and amortized period   1 year      
Rate review costs | Maximum          
Regulatory Liabilities [Line Items]          
Costs capitalized and amortized period   5 years      
Long-term maintenance contract          
Regulatory Liabilities [Line Items]          
Total regulatory assets   $ 6,504   9,134  
Regulatory assets, recovery period   5 years      
Other          
Regulatory Liabilities [Line Items]          
Total regulatory assets   $ 60,170   $ 26,285