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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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