Stock-Based Compensation - Assumptions used in Black-Scholes Option-Pricing Model (Detail) - $ / shares	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Shares subject to outstanding common stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise price (in usd per share)	$ 13.32
Share price (in usd per share)	$ 49.60
Risk free interest rate, minimum (percent)	2.30%	1.80%	1.10%
Risk free interest rate, maximum (percent)	3.00%	2.30%	2.10%
Expected term	3 years 6 months
Expected volatility	79.90%
Expected volatility, minimum (percent)	80.10%	75.10%	76.50%
Expected volatility, maximum (percent)	85.20%	85.80%	78.10%
Expected dividend yield (percent)	0.00%	0.00%	0.00%
Shares subject to outstanding common stock options \| Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term	5 years 3 months 18 days	5 years 1 month 6 days	5 years 1 month 6 days
Shares subject to outstanding common stock options \| Maximum [Member]
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
Employee Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk free interest rate, minimum (percent)	2.10%	1.10%	0.50%
Risk free interest rate, maximum (percent)	2.70%	1.60%	0.80%
Expected volatility, minimum (percent)	44.70%	54.80%	59.50%
Expected volatility, maximum (percent)	113.10%	152.80%	71.30%
Expected dividend yield (percent)	0.00%	0.00%	0.00%
Employee Stock [Member] \| Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term	6 months	6 months	6 months
Employee Stock [Member] \| Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term	1 year	1 year	1 year

Agreed-upon price for the exchange of the underlying asset relating to the share-based payment award.

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.

+ References

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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.

+ References

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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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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.

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No definition available.

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- Definition

Price of a single share of a number of saleable stocks of a company.

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No definition available.

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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.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (f)(2)(i)
-URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 14.D.2)
-URI http://asc.fasb.org/extlink&oid=115993241&loc=d3e301413-122809

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