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Pension and other post-retirement benefits - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	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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Employee Benefits Disclosure [Line Items]

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Amount of actuarial present value of benefits attributed to employee service rendered, excluding assumptions about future compensation level.

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-Publisher FASB
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Amount, before tax, after reclassification adjustment, of (increase) decrease in accumulated other comprehensive income from prior service cost (credit) of defined benefit plan.

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Amount of contribution expected to be received by defined benefit plan from employer in next fiscal year following current fiscal year.

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