Stock Compensation Plans - Ranges of Underlying Assumptions Used to Determine Fair Value of Stock Options Granted to Employees and Non-Employees (Detail) - $ / shares	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Employees and Directors
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate, minimum	1.50%	1.50%	1.70%
Risk-free interest rate, maximum	2.00%	1.70%	1.90%
Expected dividend yield	0.00%	0.00%	0.00%
Expected volatility, minimum	82.00%	77.00%	73.00%
Expected volatility, maximum	85.00%	86.00%	92.00%
Employees and Directors \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term of options (years)	5 years 10 months 24 days	5 years 10 months 24 days	5 years 9 months 18 days
Range of common stock value (usd per share)	$ 2.75	$ 5.90	$ 4.57
Employees and Directors \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term of options (years)	6 years 1 month 6 days	6 years	6 years
Range of common stock value (usd per share)	$ 5.80	$ 8.97	$ 26.71
Non-Employees
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate, minimum	1.00%	0.45%	0.10%
Risk-free interest rate, maximum	2.10%	1.93%	1.93%
Expected dividend yield	0.00%	0.00%	0.00%
Expected volatility, minimum	66.00%	77.00%	56.00%
Expected volatility, maximum	84.00%	97.00%	94.00%
Non-Employees \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term of options (years)	6 months	2 months 12 days	3 months 18 days
Range of common stock value (usd per share)	$ 2.90	$ 4.35	$ 4.04
Non-Employees \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term of options (years)	4 years 6 months	5 years 6 months	6 years
Range of common stock value (usd per share)	$ 5.95	$ 9.07	$ 25.01

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.

+ Details

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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.

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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://www.xbrl.org/2003/role/presentationRef
-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=96867065&loc=d3e5070-113901

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-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=27013229&loc=d3e301413-122809

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