Share-based compensation - Fair Value and Valuation Assumptions (Details) - $ / shares	9 Months Ended
Sep. 26, 2020	Sep. 28, 2019
—SARs
Share-based Compensation Arrangement by Share-based Payment Award
Assumed fair value exercise price (usd per share)	$ 4.59	$ 5.88
Expected volatility (percentage)	37.70%	31.90%
Expected option life (years)	6 years	6 years
Risk-free interest rate (percentage)	1.25%	2.51%
—Share options
Share-based Compensation Arrangement by Share-based Payment Award
Assumed fair value exercise price (usd per share)	$ 4.78	$ 5.88
Expected volatility (percentage)	37.60%	31.90%
Expected option life (years)	6 years	6 years
Risk-free interest rate (percentage)	1.33%	2.51%
—Premium-priced options
Share-based Compensation Arrangement by Share-based Payment Award
Assumed fair value exercise price (usd per share)		$ 5.65
Expected volatility (percentage)		31.90%
Expected option life (years)		7 years
Risk-free interest rate (percentage)		2.53%
—RSUs
Share-based Compensation Arrangement by Share-based Payment Award
Assumed fair value exercise price (usd per share)	$ 11.78	$ 16.28
—PRSUs
Share-based Compensation Arrangement by Share-based Payment Award
Assumed fair value exercise price (usd per share)	$ 14.41	$ 20.07
Expected volatility (percentage)	40.40%	32.80%
Risk-free interest rate (percentage)	1.29%	2.48%

Agreed-upon price for the exchange of the underlying asset relating to the share-based payment award.

No definition available.

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- Definition

The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

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- Definition

The risk-free interest rate assumption that is used in valuing an option on its own shares.

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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

Expected term of award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (f)(2)(i)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 14.D.2)
-URI http://asc.fasb.org/extlink&oid=115993241&loc=d3e301413-122809

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