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Stock-based Compensation - Additional Information (Details) - USD ($) $ in Thousands |  |  |  | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | --- | --- | --- | ---
May 13, 2019 | Apr. 10, 2019 | Sep. 10, 2018 | Sep. 30, 2019 | Sep. 30, 2018 | Sep. 30, 2019 | Sep. 30, 2018 | Dec. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |
Number of shares exercised |  |  |  |  |  | 85,559 |  |
Compensation expense |  |  |  | $ 307 | $ 63 | $ 5,911 | $ 129 |
Accelerated vesting of options |  |  |  |  |  | 1,139,418 |  |
Unamortized stock-based compensation cost related to unvested stock options |  |  |  | $ 2,473 |  | $ 2,473 |  |
Unamortized stock-based compensation cost, weighted average period of recognition |  |  |  |  |  | 3 years |  |
IPO |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |
Compensation expense |  |  |  |  |  | $ 287 |  |
Chief Financial Officer | IPO |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |
Accelerated vesting of options |  |  | 201,666 |  |  |  |  |
Common Stock |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |
Number of shares exercised |  |  |  | 13,525 |  | 85,559 | 9,977 |
Common Stock | Chief Executive Officer |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |
Restricted stock award, fully vested shares | 383,197 |  |  |  |  |  |  |
Restricted stock award, net shares issued | 210,758 |  |  |  |  |  |  |
Restricted stock award, shares withheld to satisfy tax obligations | 172,439 |  |  |  |  |  |  |
Tax obligations recorded as reduction in additional paid in capital | $ 1,897 |  |  |  |  |  |  |
Restricted Stock Unit |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |
Aggregate restricted stock units, granted |  | 128,000 |  |  |  |  |  |
Aggregate restricted stock units, outstanding |  |  |  | 90,000 |  | 90,000 |  |
Restricted Stock Award |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |
Compensation expense |  |  |  |  |  | $ 4,215 |  |
2012 Equity Incentive Plan |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |
Number of shares available to be issued |  |  |  | 0 |  | 0 |  |
Expiration period |  |  |  |  |  | 10 years |  |
Number of shares exercised |  |  |  |  |  | 0 |  | 0
2012 Equity Incentive Plan | Minimum |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |
Vesting period |  |  |  |  |  | 2 years |  |
2012 Equity Incentive Plan | Maximum |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |
Vesting period |  |  |  |  |  | 4 years |  |
2012 Equity Incentive Plan | Options and SARs | Minimum |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |
Exercise price options granted from fair value common stock, percent |  |  |  |  |  | 100.00% |  |
2012 Equity Incentive Plan | Granted to Ten Percent Stockholders | Minimum |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |
Exercise price options granted from fair value common stock, percent |  |  |  |  |  | 110.00% |  |
2012 Equity Incentive Plan | Granted to Ten Percent Stockholders | Maximum |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |
Vesting period |  |  |  |  |  | 5 years |  |
2019 Equity Incentive Plan |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |
Number of shares available to be issued |  |  |  | 2,011,449 |  | 2,011,449 |  |
2019 Employee Stock Purchase Plan |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |
Common stock, number of shares available for sale |  |  |  | 337,007 |  | 337,007 |  |
Period in which reserved shares will increase annually |  |  |  |  |  | 10 years |  |
Increase in common stock reserved for issuance as a percentage of total number of shares of capital stock outstanding on the last day of the prior calendar year |  |  |  | 1.00% |  | 1.00% |  |
Increase In common stock reserved for issuance of number of shares of capital stock outstanding on last day of prior calendar year |  |  |  | 500,000 |  | 500,000 |  |
Common stock reserved for issuance description |  |  |  |  |  | Additionally, the number of shares of common stock reserved for issuance under the 2019 Employee Stock Purchase Plan will automatically increase on January 1 of each calendar year for 10 years, starting January 1, 2020, and ending on, and including, January 1, 2029, in an amount equal to the lesser of 1% of the total number of shares of capital stock outstanding on December 31st of the prior calendar year, and (ii) 500,000 shares, unless the board of directors or compensation committee determines prior to such date that there will be a lesser increase, or no increase. |  |
Number of shares issued |  |  |  |  |  | 0 |  |
2019 Plan |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |
Common stock, number of shares available for sale |  |  |  | 1,885,039 |  | 1,885,039 |  |
Period in which reserved shares will increase annually |  |  |  |  |  | 10 years |  |
Increase in common stock reserved for issuance as a percentage of total number of shares of capital stock outstanding on the last day of the prior calendar year |  |  |  | 5.00% |  | 5.00% |  |
Common stock reserved for issuance description |  |  |  |  |  | 1,885,039 shares of common stock are reserved for future issuance under the 2019 Equity Incentive Plan, plus the number of shares subject to outstanding stock options or other stock awards that were granted under the 2012 Option Plan that are forfeited, terminated, expire or are otherwise not issued. Additionally, the number of shares of common stock reserved for issuance under the 2019 Equity Incentive Plan will automatically increase on January 1 of each calendar year for 10 years, starting January 1, 2020 and ending on and including January 1, 2029, in an amount equal to 5% of the total number of shares of capital stock outstanding on December 31 of the prior calendar year, unless the board of directors or compensation committee determines prior to the date of increase that there will be a lesser increase, or no increase. |  |

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