Stock-Based Compensation - Summary of Weighted-Average Assumptions Used in the Black-Scholes Option Pricing Model to Determine the Fair Value of the Non-Employee Stock Option Grants (Detail)	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Share Based Goods And Nonemployee Services Transaction [Abstract]
Risk-free interest rate	1.97%	1.35%	1.97%	1.28%
Expected volatility	74.00%	59.70%	74.00%	63.00%
Expected term (in years)	6 years 29 days	6 years 29 days	6 years 29 days	6 years 29 days
Expected dividend yield	0.00%	0.00%	0.00%	0.00%

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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 expected term of a nonvested share or option award issued to other than an employee.

+ References

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- Definition

Period an equity-based award is expected to be outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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- Definition

The estimated measure of the percentage amount by which a share price is expected to fluctuate during the expected term of a nonvested share or option award issued to other than an employee.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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- Definition

The risk-free interest rate assumption that is used in valuing a nonvested share or option award issued to other than an employee.

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-Publisher FASB
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