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Stock-Based Compensation - Schedule of Assumptions to Estimate Fair Value of Stock Options for Employees and Non-Employees Using Black-Scholes Option Pricing Model (Details) - Options Issued and Outstanding - $ / shares	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Employees and Directors
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)	6 years 1 month 6 days
Expected volatility, minimum	92.40%	89.80%	92.40%	75.90%
Expected volatility, maximum	92.60%	90.60%	104.80%	90.60%
Risk-free interest rate, minimum	1.40%	2.80%	1.40%	2.40%
Risk-free interest rate, maximum	1.80%	2.90%	2.60%	2.90%
Expected dividend yield	0.00%	0.00%	0.00%	0.00%
Non-employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Fair value of common stock				$ 15.45
Expected term (in years)				10 years
Expected volatility, minimum	89.70%		89.70%
Expected volatility, maximum	90.80%		94.20%
Risk-free interest rate, minimum	1.70%		1.70%
Risk-free interest rate, maximum	1.90%		2.60%
Expected dividend yield	0.00%		0.00%	0.00%
Expected volatility				77.90%
Risk-free interest rate				2.70%
Minimum [Member] | Employees and Directors
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Fair value of common stock	$ 9.36	$ 9.03	$ 6.97	$ 8.57
Expected term (in years)		5 years 10 months 24 days	5 years 6 months	5 years 9 months 18 days
Minimum [Member] | Non-employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Fair value of common stock	$ 8.84		$ 6.82
Expected term (in years)	7 years 8 months 12 days		7 years 8 months 12 days
Maximum | Employees and Directors
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Fair value of common stock	$ 10.66	$ 13.94	$ 10.66	$ 15.45
Expected term (in years)		6 years 1 month 6 days	6 years 1 month 6 days	6 years 7 months 6 days
Maximum | Non-employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Fair value of common stock	$ 10.26		$ 10.26
Expected term (in years)	9 years 6 months		10 years

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- Definition

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

+ References

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The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term.

+ 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)(iii)
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The estimated measure of the 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.

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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.

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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.

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

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

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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.

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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.

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