Fair Values of Stock Options Estimated Using Black-Scholes Valuation Model (Detail) - Employee Stock Option - $ / shares	3 Months Ended	6 Months Ended
Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Share Based Compensation Arrangement By Share Based Payment Award Fair Value Assumptions Black Scholes Method Used [Line Items]
Risk-free rate	0.96%	0.40%	0.74%	1.29%
Expected term (in years)	5 years 8 months 12 days	5 years 10 months 24 days	6 years	5 years 7 months 6 days
Dividend yield	0.00%	0.00%	0.00%	0.00%
Volatility	92.79%	115.20%	93.64%	111.27%
Weighted-average fair value of stock options granted	$ 3.37	$ 5.25	$ 4.45	$ 5.50

Share based compensation arrangement by share based payment award fair value assumptions Black-Scholes method used.

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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.

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- Definition

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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- Definition

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

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- Definition

The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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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.

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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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-Section S99
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-URI http://asc.fasb.org/extlink&oid=115993241&loc=d3e301413-122809

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