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Stock-Based Compensation - Additional Information (Detail) $ in Millions | 3 Months Ended |
--- | --- | ---
Mar. 31, 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 | 21,300,000 |  |
Stock-based compensation, award term | 10 years |  |
Performance-based stock awards, outstanding | 45,000 |  |
Outstanding options and other stock based awards | 17,722,000 | 17,560,000 |
Number of shares available for future issuance | 1,200,000 |  |
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 | 2,399,000 | 2,001,000 | [1]
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 |  |
[1] | Includes shares issuable under performance-based restricted stock unit awards.

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- Definition The maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested). + 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 (a)(3) -URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901 + Details
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