Stock-based compensation (Details) - USD ($)	3 Months Ended	9 Months Ended
Sep. 26, 2020	Sep. 28, 2019	Sep. 26, 2020	Sep. 28, 2019
Restricted stock units (RSUs)
Share Based Compensation Arrangement by Share Based Payment Award [Line Items]
Number of shares granted in period (in shares)	4,000	0	193,000	187,000
Weighted-average grant-date fair value (in usd per share)	$ 94.25		$ 120.35	$ 137.30
Recognized stock-based compensation costs	$ 4,883,000	$ 5,651,000	$ 14,559,000	$ 17,228,000
Recognized stock-based compensation costs, net of tax	3,614,000	$ 4,182,000	10,773,000	12,749,000
Pre-tax unrecognized compensation expense, net of forfeitures	$ 22,977,000		$ 22,977,000
Recognized expense over a weighted-average period (years)			1 year 9 months 29 days
Share-based payment arrangement, option
Share Based Compensation Arrangement by Share Based Payment Award [Line Items]
Recognized stock-based compensation costs			$ 0	$ 0

Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
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Amount, after tax, of expense for award under share-based payment arrangement.

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Amount of cost not yet recognized for nonvested award under share-based payment arrangement.

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Weighted-average period over which cost not yet recognized is expected to be recognized for award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Net number of share options (or share units) granted during the period.

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