Stock-based Compensation - Assumptions to Estimate Fair Value of Stock Option Award Using the Black-Scholes Option-Pricing Model (Detail)	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Weighted-average expected volatility	69.30%	70.50%
Expected term (in years)	6 years	6 years
Risk-free interest rate	2.50%	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

Weighted average expected volatility rate of share-based compensation awards.

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- Definition

Expected term of share-based compensation awards, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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-Name Accounting Standards Codification
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