Stock Option Plan - Summary of Assumptions Used to Value Under Stock Option Plans (Details) - Share-based Payment Arrangement, Option - $ / shares	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018
Employee and Director \| Option Plans
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Volatility, minimum	83.00%	80.69%
Volatility, maximum	84.99%	81.48%
Risk-free interest rate, minimum	1.58%	2.62%
Risk-free interest rate, maximum	2.23%	2.88%
Non-employees \| 2015 Stock Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Volatility, minimum	81.43%	78.29%
Volatility, maximum	84.85%	85.47%
Risk-free interest rate, minimum	1.40%	1.32%
Risk-free interest rate, maximum	2.67%	3.19%
Minimum \| Employee and Director \| Option Plans
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Fair value of common stock	$ 6.30	$ 1.03
Expected term (in years)	5 years 3 days	5 years 8 months 1 day
Minimum \| Non-employees \| 2015 Stock Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)	5 years 4 months 2 days	6 years 4 months 2 days
Maximum \| Employee and Director \| Option Plans
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Fair value of common stock	$ 27.53	$ 1.05
Expected term (in years)	6 years 29 days	6 years 29 days
Maximum \| Non-employees \| 2015 Stock Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)	9 years 8 months 26 days	10 years

Share-based compensation arrangement by share-based payment award, fair value assumptions, fair value of common stock.

No definition available.

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- Definition

The estimated measure of the maximum percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

+ References

No definition available.

+ Details

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- Definition

The estimated measure of the minimum percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

+ References

No definition available.

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- Definition

The maximum risk-free interest rate assumption that is used in valuing an option on its own shares.

+ References

No definition available.

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- Definition

The minimum risk-free interest rate assumption that is used in valuing an option on its own shares.

+ References

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

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- Definition

Expected term of award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (f)(2)(i)
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-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 14.D.2)
-URI http://asc.fasb.org/extlink&oid=115993241&loc=d3e301413-122809

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