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Fair Value of Financial Instruments - Auction rate securities and Contingent consideration (Details) - Auction rate securities - Significant Unobservable Inputs (Level 3) - USD ($) $ in Thousands | 3 Months Ended |
--- | --- | ---
Mar. 31, 2018 | Dec. 30, 2017
Weighted Average | Discounted cash flow |  |
Quantitative information for Level 3 Fair Value Measurements Assets |  |
Estimated yield (as a percent) | 2.22% |
Expected holding period | 10 years |
Estimated discount rate (as a percent) | 3.63% |
Assets and liabilities measured at fair value on recurring basis |  |
Quantitative information for Level 3 Fair Value Measurements Assets |  |
Fair value balance at the end of the period | $ 5,609 | $ 5,681

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