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Employee benefits - Accumulated benefit obligation in excess of plan assets (Details) - USD ($) $ in Thousands | Dec. 31, 2021 | Dec. 31, 2020
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Pension Plans |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Projected benefit obligation | $ 851,471 | $ 914,353
Accumulated benefit obligation | 851,471 | 914,353
Fair value of plan assets | 860,484 | 878,785
OPEB Plan |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Projected benefit obligation | 159,958 | 179,210
Accumulated benefit obligation | 159,958 | 179,210
Fair value of plan assets | $ 0 | $ 0

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