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Stock-Based Incentive Compensation Plans - Narrative (Details) - USD ($) $ in Thousands |  |  | 3 Months Ended | 6 Months Ended |  |
--- | --- | --- | --- | --- | --- | ---
Mar. 22, 2019 | Jan. 01, 2019 | Jun. 30, 2019 | Jun. 30, 2018 | Jun. 30, 2019 | Jun. 30, 2018 | Dec. 31, 2018 | Oct. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |
Unrecognized compensation cost |  |  | $ 7,400 |  | $ 7,400 |  |  |
Early exercise of stock options, share subject to repurchase (in shares) |  |  | 46,811 |  | 46,811 |  | 74,019 |
Liability for early exercise of unvested stock options |  |  | $ 210 |  | $ 210 |  | $ 331 |
Stock-based compensation expense |  |  | 1,814 | $ 363 | $ 3,685 | $ 754 |  |
Stock Options |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |
Unrecognized compensation cost, expected period for recognition (in years) |  |  |  |  | 2 years 10 months 25 days |  |  |
Restricted Stock Units |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |
Unrecognized compensation cost, expected period for recognition (in years) |  |  |  |  | 3 years 4 months 24 days |  |  |
Unrecognized compensation cost |  |  | $ 9,200 |  | $ 9,200 |  |  |
2018 EIP |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |
Annual increase in shares reserved as percentage of prior year-end shares outstanding (percent) |  |  | 5.00% |  | 5.00% |  |  |
Increase in number of shares of common stock reserved for issuance (in shares) |  | 1,222,538 |  |  |  |  |  |
Number of shares of common stock available for future grants (in shares) |  |  | 2,591,917 |  | 2,591,917 |  |  |
2018 ESPP |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |
Increase in number of shares of common stock reserved for issuance (in shares) | 244,507 |  |  |  |  |  |  |
Unrecognized compensation cost |  |  | $ 300 |  | $ 300 |  |  |
Number of shares reserved for issuance (in shares) |  |  |  |  |  |  |  | 515,307
Accrued compensation and related expenses for employee payroll deductions |  |  | 200 |  | 200 |  | $ 400 |
Stock-based compensation expense |  |  | $ 200 |  | $ 400 |  |  |

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- Definition Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5047-113901 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB Topic 14.F) -URI http://asc.fasb.org/extlink&oid=115993241&loc=d3e301413-122809 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(1)(i) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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