Share-based compensation - RSU and PRSU Rollforward (Details)	6 Months Ended
Jun. 27, 2020 $ / shares shares
Number of awards
Beginning balance (shares) \| shares	966,819
Granted (shares) \| shares	1,548,138
Forfeited (shares) \| shares	(90,432)
Vested (shares) \| shares	(225,441)
Ending balance (shares) \| shares	2,199,084
Weighted average grant date fair value $
Beginning balance (usd per share) \| $ / shares	$ 17.20
Granted (usd per share) \| $ / shares	12.42
Forfeited (usd per share) \| $ / shares	18.01
Vested (usd per share) \| $ / shares	16.35
Ending balance (usd per share) \| $ / shares	$ 13.89
—RSUs
Number of awards
Beginning balance (shares) \| shares	718,269
Granted (shares) \| shares	1,182,880
Forfeited (shares) \| shares	(48,274)
Vested (shares) \| shares	(225,441)
Ending balance (shares) \| shares	1,627,434
Weighted average grant date fair value $
Beginning balance (usd per share) \| $ / shares	$ 16.20
Granted (usd per share) \| $ / shares	11.81
Forfeited (usd per share) \| $ / shares	16.22
Vested (usd per share) \| $ / shares	16.35
Ending balance (usd per share) \| $ / shares	$ 12.99
—PRSUs
Number of awards
Beginning balance (shares) \| shares	248,550
Granted (shares) \| shares	365,258
Forfeited (shares) \| shares	(42,158)
Ending balance (shares) \| shares	571,650
Weighted average grant date fair value $
Beginning balance (usd per share) \| $ / shares	$ 20.07
Granted (usd per share) \| $ / shares	14.41
Forfeited (usd per share) \| $ / shares	20.07
Ending balance (usd per share) \| $ / shares	$ 16.45

The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

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 (c)(1)(iv)(3)
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Weighted average fair value as of the grant date of equity-based award plans other than stock (unit) option plans that were not exercised or put into effect as a result of the occurrence of a terminating event.

+ References

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-Publisher FASB
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-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
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The number of grants made 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).

+ References

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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).

+ References

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The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

+ References

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-Name Accounting Standards Codification
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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

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Per share or unit weighted-average fair value of nonvested award under share-based payment arrangement. Excludes share and unit options.

+ References

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-Publisher FASB
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The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

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-Publisher FASB
-Name Accounting Standards Codification
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The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement.

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