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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 (Detail) - Options Issued and Outstanding - $ / shares	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)		6 years 1 month 6 days
Expected volatility, minimum		93.70%	92.00%	75.90%	74.40%
Expected volatility, maximum		94.80%	104.90%	90.60%	74.50%
Risk-free interest rate, minimum		2.50%	1.40%	2.40%	1.90%
Risk-free interest rate, maximum		2.60%	2.60%	3.10%	2.20%
Expected dividend yield		0.00%	0.00%	0.00%	0.00%
Non-employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected volatility, minimum	99.60%	91.30%	89.70%	78.00%	74.60%
Expected volatility, maximum	101.70%	94.90%	99.50%	91.20%	77.00%
Risk-free interest rate, minimum	0.60%	2.40%	1.70%	2.70%	2.30%
Risk-free interest rate, maximum	0.90%	2.60%	2.80%	3.10%	2.40%
Expected dividend yield	0.00%	0.00%	0.00%	0.00%	0.00%
Minimum | Employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)			5 years 6 months	5 years 9 months 18 days	5 years 10 months 24 days
Fair value of common stock		$ 8.53	$ 1.27	$ 8.57	$ 0.79
Minimum | Non-employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)	7 years 2 months 12 days	8 years 2 months 12 days	7 years 4 months 24 days	8 years 4 months 24 days	9 years 4 months 24 days
Fair value of common stock	$ 0.96	$ 6.82	$ 1.23	$ 8.62	$ 0.79
Maximum | Employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)			6 years 1 month 6 days	6 years 2 months 12 days	6 years 2 months 12 days
Fair value of common stock		$ 8.9	$ 10.66	$ 15.45	$ 9.33
Maximum | Non-employees
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)	9 years	10 years	10 years	10 years	10 years
Fair value of common stock	$ 1.07	$ 8.61	$ 10.26	$ 15.45	$ 10.28

X

- Definition

Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions Fair Value Of Common Stock

+ References

No definition available.

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- Definition

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)
-URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901

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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.

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

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The minimum 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.

+ 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
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-Publisher FASB
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-Subparagraph (SAB Topic 14.D.2)
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