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Stock Incentive Plans - Employee Stock Purchase Plan (Details) - shares |  |  | 1 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | ---
Jan. 01, 2018 | Jan. 01, 2017 | Jun. 30, 2015 | Dec. 31, 2017 | Dec. 31, 2016
Stock-Based Compensation |  |  |  |  |
Common Stock, Issued, (in shares) |  |  |  | 23,645,247 | 18,817,572
2015 Plan |  |  |  |  |
Stock-Based Compensation |  |  |  |  |
Shares available for grant (in shares) |  |  |  | 1,068,287 |
2015 Plan | Annual minimum increase |  |  |  |  |
Stock-Based Compensation |  |  |  |  |
Shares available for grant (in shares) |  |  |  | 1,297,334 |
Percentage of outstanding Common Stock available for issuance (as a percent) |  |  |  | 4.00% |
Potential increase in shares available for issuance as determined by the Board of Directors (in shares) | 945,810 | 752,702 |  |  |
Common Stock | 2015 ESPP |  |  |  |  |
Stock-Based Compensation |  |  |  |  |
Common Stock, Issued, (in shares) |  |  |  | 0 |
Shares available for grant (in shares) |  |  | 182,352 |  |
Common Stock | 2015 ESPP | Annual minimum increase |  |  |  |  |
Stock-Based Compensation |  |  |  |  |
Percentage of outstanding Common Stock available for issuance (as a percent) |  |  | 1.00% |  |
Potential increase in shares available for issuance as determined by the Board of Directors (in shares) | 236,452 | 188,175 | 364,705 |  |

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- Definition Total number of common shares of an entity that have been sold or granted to shareholders (includes common shares that were issued, repurchased and remain in the treasury). These shares represent capital invested by the firm's shareholders and owners, and may be all or only a portion of the number of shares authorized. Shares issued include shares outstanding and shares held in the treasury. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(29)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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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 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 Maximum number of shares that may be issued in accordance with the plan as a proportion of outstanding capital stock. + References No definition available. + Details
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