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Stock-Based Compensation - Weighted Average Grant-Date Fair Value of Stock Options and ESPP Purchases (Details) - $ / shares | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 28, 2018 | Sep. 29, 2017 | Sep. 28, 2018 | Sep. 29, 2017
Stock options |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Weighted average grant-date fair value (in dollars per share) | $ 8.67 | $ 11.75 | $ 9.13 | $ 10.32
ESPP |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Weighted average grant-date fair value (in dollars per share) | $ 6.19 | $ 6.85 | $ 6.96 | $ 5.29

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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- Definition The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology. + References 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 (d)(1) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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