Changes in shareholders' equity - Reconciliation of basic to diluted weighted average common shares outstanding (Details) - shares shares in Thousands	3 Months Ended	9 Months Ended
Sep. 25, 2016	Sep. 27, 2015	Sep. 25, 2016	Sep. 27, 2015	Dec. 31, 2015
Debt Instrument [Line Items]
Basic (in shares)	44,045	41,597	43,081	41,542
Dilutive effect of share-based awards (in shares)	639	511	574	483
Dilutive effect of 3.875% Convertible Notes and warrants (in shares)	2,762	6,424	4,169	5,944
Diluted (in shares)	47,446	48,532	47,824	47,969
Senior Notes \| 3.875% Convertible Senior Subordinated Notes due 2017
Debt Instrument [Line Items]
Debt instrument, stated interest rate	3.875%	3.875%	3.875%	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))
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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.

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

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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
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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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