Stock-based compensation (Details) - Restricted Stock Units (RSUs) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
Mar. 28, 2020	Mar. 30, 2019
Share Based Compensation Arrangement by Share Based Payment Award [Line Items]
Number of shares granted in period (in shares)	188	169
Weighted-average grant-date fair value (in usd per share)	$ 120.94	$ 137.71
Recognized stock-based compensation costs	$ 5,041	$ 5,789
Recognized stock-based compensation costs, net of tax	3,731	$ 4,283
Pre-tax unrecognized compensation expense, net of forfeitures	$ 17,815
Recognized expense over a weighted-average period (years)	2 years 3 months 7 days

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

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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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