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Stock Incentive Plan - Additional Information (Details) $ / shares in Units, $ in Thousands |  | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | ---
Dec. 01, 2017 shares | Dec. 31, 2018 USD ($) vote $ / shares shares | Dec. 31, 2017 USD ($) $ / shares shares | Dec. 31, 2016 USD ($) $ / shares shares | Dec. 31, 2011 | Dec. 31, 2015 $ / shares | Dec. 31, 2010 $ / shares shares | May 31, 2010 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |
Preferred stock, shares authorized (in shares) |  | 5,000,000 | 5,000,000 |  |  |  |  |
Preferred stock, par value (in dollars per share) | $ / shares |  | $ 0.0001 | $ 0.0001 |  |  |  |  |
Granted (in shares) |  | 518,000 | 390,000 | 665,000 |  |  |  |
Total intrinsic value of options exercised | $ |  | $ 5,588 | $ 808 | $ 575 |  |  |  |
Exercise of stock options, net (in shares) |  | 909,000 | 401,000 | 321,000 |  |  |  |
Exercised (in usd per share) | $ / shares |  | $ 7.367 | $ 4.935 | $ 3.286 |  |  |  |
Exercise of stock options, net | $ |  | $ 6,696 | $ 1,977 | $ 1,054 |  |  |  |
Cash received from option exercises under all stock-based payment arrangements, net | $ |  | $ 7,197 | $ 1,977 | $ 1,054 |  |  |  |
Weighted average exercise price (in usd per share) | $ / shares |  | $ 8.050 | $ 7.367 | $ 7.300 |  | $ 7.740 |  |
Stock Options |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |
Options vesting period |  |  |  |  | 5 years |  |  |
Ratable vesting percentage |  | 33.33% |  |  |  |  |  |
Exercise term upon termination |  |  |  | 90 days |  |  |  |
Weighted average exercise price (in usd per share) | $ / shares |  | $ 5.20 | $ 2.93 | $ 2.60 |  |  |  |
Stock-based compensation expense | $ |  | $ 1,258 | $ 1,293 | $ 1,462 |  |  |  |
Non-vested stock options unrecognized compensation expense | $ |  | $ 3,107 |  |  |  |  |  |
Non-vested stock options unrecognized compensation expense, weighted-average period of recognition |  | 2 years 1 month 18 days |  |  |  |  |  |
Stock Options | Minimum |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |
Options vesting period |  | 3 years |  |  |  |  |  |
Class A Common Stock |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |
Common stock, shares authorized (in shares) |  | 500,000,000 | 500,000,000 |  |  |  |  |
Common stock, par value (in dollars per share) | $ / shares |  | $ 0.0001 | $ 0.0001 |  |  |  |  |
Number of votes per share | vote |  | 1 |  |  |  |  |  |
Class B Common Stock |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |
Common stock, shares authorized (in shares) |  | 144,000,000 | 144,000,000 |  |  |  |  |
Common stock, par value (in dollars per share) | $ / shares |  | $ 0.0001 | $ 0.0001 |  |  |  |  |
Number of votes per share | vote |  | 5 |  |  |  |  |  |
Common stock, shares issuable upon conversion (in shares) |  | 1 |  |  |  |  |  |
Preferred Stock |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |
Preferred stock, shares authorized (in shares) |  | 5,000,000 |  |  |  |  |  |
Preferred stock, par value (in dollars per share) | $ / shares |  | $ 0.0001 |  |  |  |  |  |
2000 Stock Incentive Plan |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |
Common stock, par value (in dollars per share) | $ / shares |  |  |  |  |  |  | $ 0.0001 |
Shares reserved for future issuance (in shares) |  |  |  |  |  |  | 28,500,000 |
2000 Stock Incentive Plan | Stock Options | Maximum |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |
Options vesting period |  | 10 years |  |  |  |  |  |
2010 Stock Incentive Plan | Stock Options |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |
Shares available for grant (in shares) |  | 6,833,000 |  |  |  |  |  |
Options vesting period |  | 10 years |  |  |  |  |  |
2010 Stock Incentive Plan | Class A Common Stock |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |
Shares reserved for future issuance (in shares) |  |  |  |  |  |  |  | 10,000,000
Granted (in shares) |  | 3,887,000 |  |  |  |  |  |
2017 Employee Stock Purchase Plan | Stock Options |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |
Number of allocated shares (up to) (in shares) | 200,000 |  |  |  |  |  |  |
Discount from fair value of stock | 5.00% |  |  |  |  |  |  |
Stock issued during period (in shares) |  | 51,000 |  |  |  |  |  |

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