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Stock Option Plan - Narrative (Details) - USD ($) | 1 Months Ended | 3 Months Ended | 9 Months Ended |
--- | --- | --- | --- | ---
Aug. 31, 2018 | Mar. 31, 2017 | Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017 | Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Forgiveness of notes receivable |  |  |  |  | $ 0 | $ 437,000 |
Early exercise of stock options, share subject to repurchase (in shares) |  |  | 78,757 |  | 78,757 |  | 16,117
Liability for early exercise of unvested stock options |  |  | $ 359,000 |  | $ 359,000 |  | $ 65,000
Stock options granted (in shares) |  |  |  |  | 73,783 |  |
Weighted-average exercise price of stock options granted (in dollars per share) |  |  | $ 9.88 |  | $ 5.51 |  |
Unrecognized compensation cost |  |  | $ 1,900,000 |  | $ 1,900,000 |  |
Minimum |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Exercise price of option as percent of fair market value when individual owns more than 10% of shares outstanding (percent) |  |  |  |  | 110.00% |  |
Incentive Stock Options | Minimum |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Exercise price of option as percent of fair market value (percent) |  |  |  |  | 100.00% |  |
Nonqualified Stock Options | Minimum |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Exercise price of option as percent of fair market value (percent) |  |  |  |  | 85.00% |  |
Stock Options |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Expiration period (in years) |  |  |  |  | 10 years |  |
Expiration period when individual holds more than 10% of shares outstanding (in years) |  |  |  |  | 5 years |  |
Award vesting period (in years) |  |  |  |  | 4 years |  |
Stock options granted (in shares) |  |  | 19,998 |  |  |  |
Unrecognized compensation cost, expected period for recognition (in years) |  |  |  |  | 2 years 1 month 6 days |  |
Stock-based compensation expense |  |  | $ 328,000 | $ 343,000 | $ 1,081,000 | $ 1,038,000 |
Performance Shares |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Awards granted in period (in shares) |  | 564,098 |  |  |  |  |
Weighted-average grant date fair value (in dollars per share) |  | $ 5.94 |  |  |  |  |
Stock-based compensation expense |  |  |  |  | $ 0 |  |
Performance Shares | Vesting Tranche One |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Award vesting period (in years) |  | 4 years |  |  |  |  |
Awards granted in period (in shares) |  | 408,544 |  |  |  |  |
Performance Shares | Vesting Tranche Two |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Award vesting period (in years) |  | 3 years |  |  |  |  |
Awards granted in period (in shares) |  | 155,554 |  |  |  |  |
Restricted Stock Units |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Awards granted in period (in shares) | 37,036 |  |  |  |  |  |
Weighted-average grant date fair value (in dollars per share) | $ 7.74 |  |  |  |  |  |
Restricted Stock Units | Vesting Tranche One |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Award vesting period (in years) | 1 year |  |  |  |  |  |
Awards vesting rights (percent) | 50.00% |  |  |  |  |  |
Restricted Stock Units | Vesting Tranche Two |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Award vesting period (in years) | 2 years |  |  |  |  |  |
Awards vesting rights (percent) | 50.00% |  |  |  |  |  |
2008 Stock Option Plan |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |
Number of shares authorized (in shares) |  |  |  |  |  |  | 5,350,080

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