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Stock Incentive Plans - Narrative (Details) |  | 3 Months Ended | 6 Months Ended |  |  |  |
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Jun. 21, 2016 shares | Jun. 30, 2020 USD ($) $ / shares shares | Jun. 30, 2019 USD ($) $ / shares | Jun. 30, 2020 USD ($) $ / shares shares | Jun. 30, 2019 USD ($) $ / shares shares | Dec. 31, 2019 shares | Mar. 25, 2019 shares | Sep. 25, 2018 shares | Dec. 31, 2008 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Common stock authorized and reserved for future issuance (in shares) |  | 27,757,227 |  | 27,757,227 |  | 40,169,185 |  |  |
Unrecognized compensation expense related to unvested non-employee stock options | $ |  | $ 0 |  | $ 0 |  |  |  |  |
Employee stock option grants |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Common stock authorized and reserved for future issuance (in shares) |  | 5,283,139 |  | 5,283,139 |  | 1,765,018 |  |  |
Estimated forfeitures rate |  |  |  | 10.00% |  |  |  |  |
Weighted average grant date fair value of stock options (in dollars per share) | $ / shares |  | $ 2.08 | $ 1.83 | $ 1.73 | $ 1.78 |  |  |  |
Aggregate intrinsic value of stock options exercised | $ |  |  | $ 100,000 |  | $ 100,000 |  |  |  |
Unrecognized compensation expense related to unvested employee stock options | $ |  | $ 8,100,000 |  | $ 8,100,000 |  |  |  |  |
Weighted average period for recognition |  |  |  | 2 years 3 months 18 days |  |  |  |  |
Non-employee stock options |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Weighted average grant date fair value of stock options (in dollars per share) | $ / shares |  |  | $ 1.46 |  | $ 0.65 |  |  |  |
Unvested restricted stock units |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Common stock authorized and reserved for future issuance (in shares) |  | 159,375 |  | 159,375 |  | 181,250 |  |  |
Grants in period (in shares) |  |  |  | 0 |  |  |  |  |
Vested in period (in dollars per share) | $ / shares |  |  |  | $ 6.03 |  |  |  |  |
Grants in period (in dollars per share) | $ / shares |  |  |  | $ 0 |  |  |  |  |
The 2008 Plan and the 2016 Plan | Employee stock option grants |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Outstanding, Number |  | 6,472,944 |  | 6,472,944 |  |  |  |  |
Issuance of common stock upon exercise of options (in shares) |  |  |  | 42,628 |  |  |  |  |
Aggregate intrinsic value of stock options exercised | $ |  | $ 3,381,000 |  | $ 3,381,000 |  |  |  |  |
The 2008 Plan and the 2016 Plan | Non-employee stock options |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Outstanding, Number |  | 473,073 |  | 473,073 |  |  |  |  |
Issuance of common stock upon exercise of options (in shares) |  |  |  | 0 |  |  |  |  |
Aggregate intrinsic value of stock options exercised | $ |  | $ 72,000 |  | $ 72,000 |  |  |  |  |
2008 Plan |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Number of shares authorized for grants (in shares) |  |  |  |  |  |  |  |  | 2,213,412
2008 Plan | Employee stock option grants |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Award vesting term |  |  |  | 4 years |  |  |  |  |
Award expiration term |  |  |  | 10 years |  |  |  |  |
2016 Plan |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Number of shares authorized for grants (in shares) |  | 1,210,256 |  | 1,210,256 |  |  |  |  |
Automatic increase in the number of shares that may be issued (as a percent) |  |  |  | 0.04 |  |  |  |  |
Number of shares authorized, increase (in shares) |  |  |  | 3,453,022 | 898,871 |  |  |  |
Common stock authorized and reserved for future issuance (in shares) |  | 2,651,062 |  | 2,651,062 |  |  |  |  |
2016 Plan | Employee stock option grants |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Award vesting term |  |  |  | 4 years |  |  |  |  |
Award expiration term |  |  |  | 10 years |  |  |  |  |
2016 Plan | Employee stock option grants | Minimum |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Combined voting power of participants (as a percent) |  |  |  | 10.00% |  |  |  |  |
2016 Plan | Employee stock option grants | Maximum |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Combined voting power of participants (as a percent) |  |  |  | 10.00% |  |  |  |  |
2016 Plan | Employee stock option grants | Tranche One | Minimum |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Exercise price of incentive stock options (not less than) |  |  |  | 100.00% |  |  |  |  |
2016 Plan | Employee stock option grants | Tranche Two | Minimum |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Exercise price of incentive stock options (not less than) |  |  |  | 110.00% |  |  |  |  |
Employment Inducement Incentive Award Plan |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Number of shares authorized for grants (in shares) |  | 1,100,000 |  | 1,100,000 |  |  |  |  |
Common stock authorized and reserved for future issuance (in shares) |  |  |  |  |  |  | 2,000,000 | 1,175,000 |
Employment Inducement Incentive Award Plan | Unvested restricted stock units |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Weighted average period for recognition |  |  |  | 2 years 4 months 24 days |  |  |  |  |
Unrecognized compensation expense | $ |  | $ 600,000 |  | $ 600,000 |  |  |  |  |
Employment Inducement Incentive Award Plan | Unvested restricted stock units | Executive Officer |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Grants in period (in dollars per share) | $ / shares |  | $ 2.92 |  |  |  |  |  |  |
Vested in period, fair value | $ |  | $ 100,000 |  |  |  |  |  |  |
ESPP |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Automatic increase in the number of shares that may be issued (as a percent) | 0.01 |  |  |  |  |  |  |  |
Number of shares authorized, increase (in shares) |  |  |  | 863,254 | 224,717 |  |  |  |
Common stock authorized and reserved for future issuance (in shares) | 173,076 | 1,532,077 |  | 1,532,077 |  |  |  |  |
Exercise price of incentive stock options (not less than) | 85.00% |  |  |  |  |  |  |  |

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