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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, 2020 | Dec. 31, 2019
Employee Benefits Disclosure [Line Items] |  |
Defined contribution pension plan cost | $ 9,672 | $ 8,798
Accumulated benefit obligation for pension plan | 1,080,685 | 526,517
Defined benefit plan, amounts recognized in other comprehensive income (loss), net prior service cost (credit) |  | $ (7,798)
Pension Plans |  |
Employee Benefits Disclosure [Line Items] |  |
Expected employer contributions for next year | 28,104 |
Other Postretirement Benefit Plans, Defined Benefit |  |
Employee Benefits Disclosure [Line Items] |  |
Expected employer contributions for next year | $ 11,398 |

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- Definition Amount of contribution expected to be received by defined benefit plan from employer in next fiscal year following current fiscal year. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 55 -Paragraph 18 -URI http://asc.fasb.org/extlink&oid=118257860&loc=d3e4587-114921 + Details
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