Stock-Based Compensation - Assumptions used in Black-Scholes Option-Pricing Model (Detail) - $ / shares	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average fair value of option on grant date (in usd per share)	$ 32.64	$ 30.87	$ 7.43
Shares subject to outstanding common stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk free interest rate, minimum (percent)	1.40%	2.30%	1.80%
Risk free interest rate, maximum (percent)	2.60%	3.00%	2.30%
Expected volatility, minimum (percent)	73.50%	80.10%	75.10%
Expected volatility, maximum (percent)	82.30%	85.20%	85.80%
Expected dividend yield (percent)	0.00%	0.00%	0.00%
Shares subject to outstanding common stock options \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term	5 years 3 months 18 days	5 years 3 months 18 days	5 years 1 month 6 days
Shares subject to outstanding common stock options \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term	6 years 1 month 6 days	6 years 1 month 6 days	6 years 1 month 6 days

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 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.

+ References

No definition available.

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- Definition

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.

+ References

No definition available.

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- Definition

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

+ References

No definition available.

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- Definition

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

+ References

No definition available.

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- Definition

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.

+ References

No definition available.

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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.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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-Topic 718
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