Stock-Based Compensation - Additional Information (Detail) $ in Millions	6 Months Ended
Jun. 30, 2019 USD ($) Period shares	Dec. 31, 2018 shares
Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation, option to be granted at percentage of fair value of common stock	85.00%
Employee Stock Purchase Plan, offering period	12 months
Number of purchase periods within each offering period \| Period	2
Aggregate number of shares of common stock reserved for future issuance	700,000
2008 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation, option to be granted at percentage of fair value of common stock	100.00%
Aggregate number of shares of common stock reserved for future issuance	31,800,000
Stock-based compensation, award term	10 years
Performance-based stock awards, outstanding	45,000
Outstanding options and other stock based awards	17,680,000	17,560,000
Number of shares available for future issuance	9,900,000
2008 Equity Incentive Plan \| Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation, vesting period	4 years
2008 Equity Incentive Plan \| Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of unvested Restricted Stock Units	4,192,000	2,001,000
2008 Equity Incentive Plan \| Performance-based Stock or Cash Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Employee Stock Purchase Plan, authorized shares for issuance	500,000
Stock option plan granted on cash award \| $	$ 1.0
2008 Equity Incentive Plan \| Minimum \| Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation, vesting period	3 years
2008 Equity Incentive Plan \| Maximum \| Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation, vesting period	4 years

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Aggregate number of common shares reserved for future issuance.

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Aggregate cash paid by the entity during the period to settle equity instruments granted under equity-based payment arrangements.

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Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition.

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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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The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable.

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Number of options outstanding, including both vested and non-vested options.

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Purchase price of common stock expressed as a percentage of its fair value.

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