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Pension and other post-retirement benefits - Additional Information (Detail) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2019 | Dec. 31, 2018
Employee Benefits Disclosure [Line Items] |  |
Defined contribution pension plan cost | $ 8,798 | $ 8,446
Accumulated benefit obligation for pension plan | 526,517 | 439,458
Defined benefit plan, amounts recognized in other comprehensive income (loss), net prior service cost (credit) | (7,798) | $ (1,875)
Pension Plans |  |
Employee Benefits Disclosure [Line Items] |  |
Expected employer contributions for next year | 24,140 |
Other Postretirement Benefit Plans, Defined Benefit |  |
Employee Benefits Disclosure [Line Items] |  |
Expected employer contributions for next year | $ 5,736 |

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