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Enviromental matters (Details 2) - USD ($) | 3 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2020 | Dec. 31, 2020
Portland Harbor Site |  |
Site Contingency [Line Items] |  |
Environmental matters investigative costs |  | $ 100,000,000
Environmental matters, estimated costs |  | 1,000,000,000
Missoula, MT manufactured gas plant site |  |
Site Contingency [Line Items] |  |
Total estimated costs for site remediation |  | 560,000
Environmental Matters Estimated Investigative Costs |  | 800,000
Environmental matters accrual for site remediation |  | 800,000
Incurred Costs For Site Remedial Investigation |  | 130,000
Bremerton, WA manufactured gas plant site |  |
Site Contingency [Line Items] |  |
Environmental matters accrual for site remediation |  | 17,500,000
Total estimated costs for site remedial investigation and feasibility study |  | 7,600,000
Incurred costs for site remedial investigation and feasibility study |  | 5,000,000.0
Environmental Matters Accrual For Site Remediation, Period Increase | $ 11,100,000 |
Environmental matters accrual of investigative costs |  | 2,600,000
Bellingham, WA manufactured gas plant site |  |
Site Contingency [Line Items] |  |
Site contingency, loss exposure not accrued, preferred alternative estimate |  | 9,300,000
Minimum | Bremerton, WA manufactured gas plant site |  |
Site Contingency [Line Items] |  |
Site contingency, loss exposure not accrued, best estimate |  | 13,600,000
Minimum | Bellingham, WA manufactured gas plant site |  |
Site Contingency [Line Items] |  |
Site contingency, loss exposure not accrued, best estimate |  | 8,000,000.0
Maximum | Bremerton, WA manufactured gas plant site |  |
Site Contingency [Line Items] |  |
Site contingency, loss exposure not accrued, best estimate |  | 71,000,000.0
Maximum | Bellingham, WA manufactured gas plant site |  |
Site Contingency [Line Items] |  |
Site contingency, loss exposure not accrued, best estimate |  | $ 20,400,000

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