Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jul. 01, 2017	Jun. 30, 2018	Jul. 01, 2017
Earnings Per Share
Net income	$ 14,280	$ 16,569	$ 40,685	$ 31,995
Shares used in computing basic earnings per share (in shares)	43,312	42,478	43,138	42,287
Effect of dilutive securities:
Stock-based awards (in shares)	982	700	968	817
Shares used in computing diluted earnings per share (in shares)	44,294	43,178	44,106	43,104
Earnings per share:
Basic (in dollars per share)	$ 0.33	$ 0.39	$ 0.94	$ 0.76
Diluted (in dollars per share)	$ 0.32	$ 0.38	$ 0.92	$ 0.74
Shares issued upon conversion of principal amount
Earnings per share:
Shares excluded from computation of diluted earning per share (in shares)			100

Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented.

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The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period.

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The amount of net income (loss) for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

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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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Amount, after deduction of tax, noncontrolling interests, dividends on preferred stock and participating securities; of income (loss) available to common shareholders.

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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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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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