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Stock-Based Compensation - Plan details (Details) |  |  | 12 Months Ended
--- | --- | --- | ---
Jan. 01, 2016 shares | Jan. 01, 2014 shares | Dec. 31, 2017 item $ / shares shares | Dec. 31, 2016 $ / shares shares | Dec. 31, 2015 $ / shares shares
Stock options |  |  |  |  |
Stock-based compensation |  |  |  |  |
Total number of shares of common stock available for issuance |  |  | 3,394,651 | 2,943,216 | 2,917,182
Options granted (in shares) |  |  | 1,511,100 | 1,429,500 |
Information with respect to stock options outstanding |  |  |  |  |
Exercisable options (in shares) |  |  | 2,558,941 | 1,743,765 | 1,600,351
Weighted-average exercise price per share of exercisable options (in dollars per share) | $ / shares |  |  | $ 11.06 | $ 8.87 | $ 3.99
Weighted average grant date fair value per share of options granted during the year (in dollars per share) | $ / shares |  |  | $ 16.92 | $ 10.30 | $ 10.66
Weighted-average remaining contractual life |  |  | 6 years 6 months 29 days | 7 years 9 months 26 days | 6 years 11 months 23 days
Employee Stock Purchase Plan |  |  |  |  |
Stock-based compensation |  |  |  |  |
Total number of shares of common stock available for issuance |  |  | 581,286 |  |
Initial term of plan |  |  | 2 years |  |
Second term of plan |  |  | 2 years |  |
Number of six month purchase periods | item |  |  | 4 |  |
Purchase period |  |  | 6 months |  |
Increase in shares of common stock available for issuance (in shares) |  | 313,545 | 0 | 0 | 0
Awards issued under the plan (in shares) |  |  | 292,393 | 221,486 |
Employee Stock Purchase Plan | Minimum |  |  |  |  |
Stock-based compensation |  |  |  |  |
Percentage of compensation that employees may withhold to purchase stock at a discount |  |  | 1.00% |  |
Employee Stock Purchase Plan | Maximum |  |  |  |  |
Stock-based compensation |  |  |  |  |
Percentage of compensation that employees may withhold to purchase stock at a discount |  |  | 15.00% |  |
Purchase price as percentage of stock price at the initial offering date |  |  | 85.00% |  |
Purchase price as percentage of stock price at the purchase date |  |  | 85.00% |  |
Annual percentage increase in shares of common stock available for issuance |  |  | 1.00% |  |
Annual increase in shares of common stock available for issuance (in shares) |  |  | 621,814 |  |
The 2010 Plan |  |  |  |  |
Stock-based compensation |  |  |  |  |
Total number of shares of common stock available for issuance |  |  | 0 |  |
The 2013 Plan |  |  |  |  |
Stock-based compensation |  |  |  |  |
Total number of shares of common stock available for issuance |  |  | 8,526,465 |  |
Annual percentage increase in shares of common stock available for issuance |  |  | 4.00% |  |
Increase in shares of common stock available for issuance (in shares) | 1,862,719 |  |  |  |
Awards issued under the plan (in shares) |  |  | 4,945,350 |  |

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- Definition Number of additional shares authorized for issuance under an established share-based compensation plan. + References No definition available. + Details
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- Definition The maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(iii) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition The weighted-average price as of the balance sheet date at which grantees can acquire the shares reserved for issuance on vested portions of options outstanding and currently exercisable under the stock option plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(iii) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + 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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d)(1) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition Weighted average remaining contractual term for option awards outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (e)(1) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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