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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

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