Exercise price of the warrants.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column A

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Antidilution
-URI http://asc.fasb.org/extlink&oid=6505113

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Diluted Earnings Per Share
-URI http://asc.fasb.org/extlink&oid=6510752

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Contingent Stock Agreement
-URI http://asc.fasb.org/extlink&oid=6508534

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4332-108586

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Shareholders' equity - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands, shares in Millions	3 Months Ended			9 Months Ended
	Oct. 01, 2017	Apr. 02, 2017	Sep. 25, 2016	Oct. 01, 2017	Sep. 25, 2016	Dec. 31, 2016
Shareholders Equity [Line Items]
Investment warrants, exercise price (in dollars per share)		$ 74.65
Conversion of convertible notes, shares issued upon conversion	0.5		1.3	0.5	1.9
Convertible notes - redeemable equity component	$ 0			$ 0		$ 1,824
Equity Option
Shareholders Equity [Line Items]
Weighted average antidilutive shares which were not included in the calculation of earnings per share	0.6		1.4	0.6	3.9
Convertible Note
Shareholders Equity [Line Items]
Weighted average antidilutive shares which were not included in the calculation of earnings per share	0.1		1.5	0.4	2.3