Shareholders' equity - Reconciliation of basic to diluted weighted average common shares outstanding (Details) - shares shares in Thousands	3 Months Ended	9 Months Ended
Oct. 01, 2017	Sep. 25, 2016	Oct. 01, 2017	Sep. 25, 2016
Debt Instrument [Line Items]
Basic (in shares)	45,035	44,045	44,975	43,081
Dilutive effect of share-based awards (in shares)	934	639	873	574
Dilutive effect of 3.875% Convertible Notes and warrants (in shares)	618	2,762	825	4,169
Diluted (in shares)	46,587	47,446	46,673	47,824
Senior Notes \| 3.875% Convertible Senior Subordinated Notes due 2017
Debt Instrument [Line Items]
Debt instrument, stated interest rate	3.875%		3.875%

Contractual interest rate for funds borrowed, under the debt agreement.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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Additional shares included in the calculation of diluted EPS as a result of the potentially dilutive effect of convertible debt securities using the if-converted method.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257

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Additional shares included in the calculation of diluted EPS as a result of the potentially dilutive effect of share based payment arrangements using the treasury stock method.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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The average number of shares or units issued and outstanding that are used in calculating diluted EPS or earnings per unit (EPU), determined based on the timing of issuance of shares or units in the period.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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Number of [basic] shares or units, after adjustment for contingently issuable shares or units and other shares or units not deemed outstanding, determined by relating the portion of time within a reporting period that common shares or units have been outstanding to the total time in that period.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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