Short-term investments - Additional Information (Details) - USD ($) |
12 Months Ended | |
|---|---|---|
Dec. 30, 2017 |
Dec. 31, 2016 |
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| Investments Schedule [Abstract] | ||
| Recognized gross realized gain (loss) from the sale of mutual funds | $ 0 | $ 0 |
| X | ||||||||||
- References No definition available.
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| X | ||||||||||
- Definition This item represents the difference between the gross realized gains and losses realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities. Additionally, this item would include any losses recognized for other than temporary impairments (OTTI) of the subject investments in debt and equity securities. Reference 1: http://www.xbrl.org/2003/role/presentationRef
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