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Employee benefits - Accumulated benefit obligation in excess of plan assets (Details) - USD ($) $ in Thousands | Dec. 31, 2019 | Dec. 31, 2018
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Pension Plans |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Projected benefit obligation | $ 852,551 | $ 754,558
Accumulated benefit obligation | 852,551 | 754,558
Fair value of plan assets | 799,935 | 685,823
OPEB Plan |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Projected benefit obligation | 168,681 | 153,415
Accumulated benefit obligation | 168,681 | 153,415
Fair value of plan assets | $ 0 | $ 0

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