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Stock Options - Additional Information (Details) - USD ($) $ in Millions |  | 9 Months Ended |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | ---
Mar. 12, 2019 | Sep. 30, 2019 | Dec. 31, 2018 | Sep. 30, 2018 | May 31, 2018 | Dec. 31, 2016 | Apr. 30, 2015
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |
Options outstanding |  | 9,047,630 | 7,763,464 |  |  |  |
Stock options outstanding |  | 2,115,331 |  |  |  |  |
Unrecognized compensation cost |  | $ 30.3 |  |  |  |  |
Weighted-average period for recognition of compensation cost |  | 3 years 1 month 9 days |  |  |  |  |
Intrinsic value of Stock options exercised |  | $ 5.3 |  | $ 1.0 |  |  |
2006 Plan |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |
Remaining shares available to be granted |  |  |  |  |  | 0 |
Options outstanding |  | 1,397,203 |  |  |  |  |
2015 Plan |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |
Options outstanding |  | 6,111,373 |  |  |  |  |
Common stock reserved for issuance |  |  |  |  | 8,211,980 |  | 5,270,095
2019 Plan |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |
Common stock reserved for issuance | 4,750,000 |  |  |  |  |  |
Allowed annual percentage increase in shares authorized as percentage of outstanding shares of common stock | 4.00% |  |  |  |  |  |
Maximum shares of common stock may be issued | 5,000,000 |  |  |  |  |  |
Stock options outstanding |  | 1,539,054 |  |  |  |  |
2019 ESPP |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |
Common stock reserved for issuance | 525,000 |  |  |  |  |  |
Allowed annual percentage increase in shares authorized as percentage of outstanding shares of common stock | 1.00% |  |  |  |  |  |
Maximum shares allowed to be issued under ESPP | 5,250,000 |  |  |  |  |  |
Purchase of common stock through payroll deductions expressed in percentage of fair market value | 85.00% |  |  |  |  |  |
Offering period commenced date | Oct. 21, 2019 |  |  |  |  |  |
Offering period end date | Feb. 29, 2020 |  |  |  |  |  |

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