Stock-based Compensation - Assumptions to Estimate Fair Value of Stock Option Award Using the Black-Scholes Option-Pricing Model (Detail)	9 Months Ended
Sep. 30, 2019	Sep. 30, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Weighted-average expected volatility	69.20%	71.90%
Expected term (in years)	6 years	6 years
Risk-free interest rate	2.40%	2.70%
Expected dividend yield	0.00%	0.00%

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.

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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

Rate of weighted-average expected volatility for award under share-based payment arrangement.

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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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