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Environmental matters (Details 2) - USD ($) | 9 Months Ended |
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Sep. 30, 2018 | Jan. 06, 2017
Portland Harbor Site |  |
Site Contingency [Line Items] |  |
Environmental matters investigative costs | $ 100,000,000 |
Environmental matters, estimated costs |  | $ 1,000,000,000
Eugene, OR manufactured gas plant site |  |
Site Contingency [Line Items] |  |
Total estimated costs for site remediation | 3,500,000 |
Incurred costs for site remediation | $ 400,000 |
Estimated proportional share of cleanup liability | 50.00% |
Environmental matters accrual for site remediation | $ 1,600,000 |
Bremerton, WA manufactured gas plant site |  |
Site Contingency [Line Items] |  |
Environmental matters accrual for site remediation | 6,400,000 |
Total estimated costs for site remedial investigation and feasibility study | 7,600,000 |
Incurred costs for site remedial investigation and feasibility study | 2,700,000 |
Environmental matters accrual of investigative costs | 4,900,000 |
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 | 340,000 |
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 | 6,400,000 |
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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