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Pension and other post-retirement benefits - Benefit Obligations in Excess of Plan Assets (Details) - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019
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Pension Plans |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Accumulated benefit obligation | $ 727,981 | $ 504,403
Fair value of plan assets | 578,143 | 407,074
Projected benefit obligation | 833,846 | 564,971
Fair value of plan assets | 627,601 | 407,074
Other Postretirement Benefit Plans, Defined Benefit |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Accumulated benefit obligation | 288,594 | 202,422
Fair value of plan assets | 148,496 | 133,711
Projected benefit obligation | 288,594 | 202,422
Fair value of plan assets | $ 148,496 | $ 133,711

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