Stock-based compensation (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jul. 01, 2017	Jun. 30, 2018	Jul. 01, 2017
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	127,000	153,000
Weighted-average grant-date fair value (in usd per share)	$ 204.35	$ 226.85	$ 237.94	$ 226.85
Recognized stock-based compensation costs	$ 13,645	$ 13,875	$ 21,593	$ 23,424
Recognized stock-based compensation costs, net of tax	10,097	$ 8,419	15,979	14,214
Pre-tax unrecognized compensation expense	$ 32,114		$ 32,114
Recognized expense over a weighted-average period (years)			1 year 7 months 6 days
Stock options
Share Based Compensation Arrangement by Share Based Payment Award [Line Items]
Recognized stock-based compensation costs				6
Recognized stock-based compensation costs, net of tax				$ 4

Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

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- Definition

The amount of expense, net of income tax, recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

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- Definition

Unrecognized cost of unvested share-based compensation awards.

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-Publisher FASB
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- Definition

Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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- Definition

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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- Definition

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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- Definition

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

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