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Stockholders' Equity - Additional Information (Detail) $ / shares in Units, $ in Thousands |  | 1 Months Ended | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | ---
Jun. 22, 2015 shares | Apr. 30, 2018 USD ($) $ / shares shares | Jun. 30, 2017 USD ($) $ / shares shares | Dec. 31, 2018 USD ($) Installment $ / shares shares | Dec. 31, 2017 USD ($) $ / shares shares | Dec. 31, 2016 USD ($) $ / shares shares | Nov. 10, 2017 shares | May 31, 2017 USD ($) | Dec. 07, 2016 USD ($)
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Preferred shares authorized |  |  |  | 5,000,000 |  |  |  |  |
Net proceeds from issuance of common stock, after deducting offering expenses | $ |  |  |  | $ 215,758 | $ 81,573 |  |  |  |
Stock option maximum term |  |  |  | 5 years 4 months 20 days |  |  |  |  |
Specified stockholder ownership percentage |  |  |  | 10.00% |  |  |  |  |
Purchase price of common stock percent under stock incentive plans for specified stockholder |  |  |  | 110.00% |  |  |  |  |
Stock option maximum term for specified stockholder |  |  |  | 5 years |  |  |  |  |
Share of common stock subject to a stock option or stock appreciation right |  |  |  | 100.00% |  |  |  |  |
Fixed ratio shares of common stock |  |  |  | 1.33 |  |  |  |  |
Additional equity award |  |  |  | 3,115,078 |  |  |  |  |
Intrinsic value of options exercised | $ |  |  |  | $ 27,000 | 12,300 | $ 100 |  |  |
Options outstanding, Aggregate Intrinsic Value | $ |  |  |  | 24,468 |  |  |  |  |
Options exercisable, Aggregate Intrinsic Value | $ |  |  |  | 12,702 |  |  |  |  |
Options vested and expected to vest, aggregate intrinsic value | $ |  |  |  | $ 24,500 | 71,700 | 0 |  |  |
Restricted stock units awarded |  |  |  | 346,055 |  |  |  |  |
Total payments for the employees' tax obligations to taxing authorities | $ |  |  |  | $ 254 | $ 654 | $ 776 |  |  |
Share-Based Compensation Awards, Tranche 2 [Member] |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Stock options granted vesting percentage |  |  |  | 0.33% |  |  |  |  |
Share-Based Compensation Awards, Tranche 3 [Member] |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Stock options granted vesting percentage |  |  |  | 0.33% |  |  |  |  |
2013 Stock Incentive Plan [Member] |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Percent of fair value per share of common stock on option grant date |  |  |  | 100.00% |  |  |  |  |
Stock options vesting period |  |  |  | 4 years |  |  |  |  |
Stock options expiration term |  |  |  | 10 years |  |  |  |  |
Stock reserved for issuance under plan |  |  |  | 14,100,000 |  |  |  |  |
Additional shares authorized under 2013 stock incentive plan | 5,300,000 |  |  | 4,400,000 |  |  |  |  |
Shares reserved for issuance of future awards |  |  |  | 9,491,418 |  |  |  |  |
2004 Stock Incentive Plan [Member] |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Stock reserved for issuance under plan |  |  |  | 9,700,000 |  |  |  |  |
Amended and Restated 2013 Stock Incentive Plan [Member] |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Stock reserved for issuance under plan |  |  |  |  |  |  | 1,000,000 |  |
Additional equity award |  | 0 |  |  |  |  |  |  |
2018 Stock Incentive Plan [Member] |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Percent of fair value per share of common stock on option grant date |  |  |  | 100.00% |  |  |  |  |
Stock options vesting period |  |  |  | 4 years |  |  |  |  |
Stock options expiration term |  |  |  | 10 years |  |  |  |  |
Share of common stock subject to a stock option or stock appreciation right |  |  |  | 100.00% |  |  |  |  |
Fixed ratio shares of common stock |  |  |  | 1.33 |  |  |  |  |
2010 Employee Stock Purchase Plan [Member] |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Percent of fair value per share of common stock on option grant date |  |  |  | 85.00% |  |  |  |  |
Stock reserved for issuance under plan |  |  |  | 4,600,000 |  |  |  |  |
Purchase plan offering period |  |  |  | 2 years |  |  |  |  |
Purchase plan purchase period |  |  |  | 6 months |  |  |  |  |
Shares reserved for issuance of future awards |  |  |  | 3,006,964 |  |  |  |  |
Maximum [Member] | 2013 Stock Incentive Plan [Member] |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Stock option maximum term |  |  |  | 10 years |  |  |  |  |
Maximum [Member] | 2018 Stock Incentive Plan [Member] |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Stock option maximum term |  |  |  | 10 years |  |  |  |  |
Employee Stock Options [Member] |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Shares subject to repurchase rights |  |  |  | 0 |  |  |  |  |
Employee Stock Options [Member] | 2013 Stock Incentive Plan [Member] |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Share based vesting rights |  |  |  | Options granted under the 2013 Plan generally vest over four years at a rate of 25% one year from the grant date and one thirty-sixth per month thereafter and expire ten years after the grant, or earlier upon employment termination. |  |  |  |  |
Stock options granted vesting percentage |  |  |  | 25.00% |  |  |  |  |
Employee Stock Options [Member] | 2018 Stock Incentive Plan [Member] |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Share based vesting rights |  |  |  | Generally, stock options granted under the 2018 Plan vest over four years at a rate of twenty-five percent (25%) on the one-year anniversary of the date of grant and one forty-eighth (1/48) per month thereafter and expire ten years after the date of grant, or earlier upon termination of employment or services to the Company |  |  |  |  |
Stock options granted vesting percentage |  |  |  | 25.00% |  |  |  |  |
Restricted Stock Units (RSUs) [Member] |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Share based vesting rights |  |  |  | These awards generally vest as follows: one-third of the award will vest in a series of three successive equal annual installments. |  |  |  |  |
Restricted stock units awarded |  |  |  | 346,055 | 12,600 | 60,000 |  |  |
Grant date fair value per award of restricted stock units | $ / shares |  |  |  | $ 17.87 | $ 15.85 | $ 5.16 |  |  |
Share-based compensation arrangement by share-based payment award, number of vesting installment | Installment |  |  |  | 3 |  |  |  |  |
Aggregate fair value of RSUs vested in period | $ |  |  |  | $ 600 | $ 1,200 | $ 4,800 |  |  |
Shares withheld from issuance in order to pay employee taxes |  |  |  | 20,193 | 42,243 | 165,181 |  |  |
Restricted Stock Units (RSUs) [Member] | Share-Based Compensation Awards, Tranche 1 [Member] |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Stock options granted vesting percentage |  |  |  | 0.33% |  |  |  |  |
ATM Agreement [Member] |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Common stock issuable and sellable under offering agreement | $ |  |  |  | $ 75,000 |  |  |  | $ 75,000 | $ 75,000
Common Stock [Member] |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Total shares issued |  | 14,200,000 | 11,500,000 | 14,156,500 | 12,371,149 |  |  |  |
Public offering price of common stock issued | $ / shares |  | $ 16.25 | $ 7.25 |  |  |  |  |  |
Net proceeds from issuance of common stock, after deducting offering expenses | $ |  | $ 215,800 | $ 78,100 |  |  |  |  |  |

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