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Stock-Based Compensation - Additional Information (Detail) $ / shares in Units, $ in Thousands |  | 1 Months Ended | 3 Months Ended | 9 Months Ended |
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Jul. 10, 2015 | Jul. 31, 2015 $ / shares shares | Jun. 30, 2015 | Mar. 31, 2015 $ / shares shares | Mar. 31, 2014 shares | Sep. 30, 2015 USD ($) $ / shares shares | Sep. 30, 2014 shares | Sep. 30, 2015 USD ($) $ / shares shares | Dec. 31, 2014 shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Forward stock split of its outstanding common stock | 1.8515 |  |  |  |  |  |  | 1.8515 |
Officer | Initial Public Offering |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Forward stock split of its outstanding common stock |  | 1 |  |  |  |  |  |  |
Officer | Vest over a 40-month service period beginning on April 1, 2015 | Initial Public Offering |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Share-based compensation arrangement, Award Vesting Rights, Percentage |  | 50.00% |  |  |  |  |  |  |
Officer | Vest based on the achievement of various milestones | Initial Public Offering |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Share-based compensation arrangement, Award Vesting Rights, Percentage |  | 50.00% |  |  |  |  |  |  |
Common Class A |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Stock price of warrants | $ / shares |  |  |  |  |  | $ 2.44 |  | $ 2.44 |
Forward stock split of its outstanding common stock |  |  | 1.00 |  |  |  |  |  |
2015 Equity Incentive Plan |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Shares reserved for issuance |  | 4,257,572 |  |  |  |  |  |  |
Additional Shares reserved for issuance |  | 8,439,494 |  |  |  |  |  |  |
Employee Stock Option |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Shares granted to employee |  |  |  |  |  |  |  | 5,713,899 |
Shares granted at an exercise price | $ / shares |  |  |  |  |  |  |  | $ 7.92 |
Aggregate intrinsic value of stock options exercised | $ |  |  |  |  |  | $ 882 |  |  |
Unrecognized compensation cost related to non-vested stock options | $ |  |  |  |  |  | $ 33,079 |  | $ 33,079 |
Weighted-average period for recognition |  |  |  |  |  |  |  | 2 years 1 month 2 days |
Stock option outstanding |  |  |  |  |  | 9,811,356 |  | 9,811,356 | 5,137,914
Employee Stock Option | Officer | Initial Public Offering |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Shares granted at an exercise price | $ / shares |  | $ 25.00 |  |  |  |  |  |  |
Granted option to purchase common stock |  | 1,455,450 |  |  |  |  |  |  |
Employee Stock Option | Common Class A | Officer |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Shares granted to employee |  |  |  | 1,851,500 |  |  |  |  |
Shares granted at an exercise price | $ / shares |  |  |  | $ 2.20 |  |  |  |  |
Option vesting period |  |  |  | 4 years |  |  |  |  |
Warrant Term |  |  |  | 4 years |  |  |  |  |
Stock price of warrants | $ / shares |  |  |  | $ 2.00 |  |  |  |  |
Option vested |  |  |  |  |  |  |  | 9,442,650 |
Employee Stock Option | Common Class A | Officer | Vest over a 40-month service period beginning on April 1, 2015 |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Warrants issued to purchase common shares |  |  |  | 7,406,000 |  |  |  |  |
Employee Stock Option | Common Class A | Officer | Vest based on the achievement of various milestones |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Warrants issued to purchase common shares |  |  |  | 10,183,250 |  |  |  |  |
Employee Stock Option | Common Class A | Maximum | Officer |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Warrants issued to purchase common shares |  |  |  | 17,589,250 |  |  |  |  |
Employee Stock Option | 2014 Equity Incentive Plan | Common Class A |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Common stock reserved for issuance |  |  |  |  | 11,109,000 |  |  |  |
Employee Stock Option | 2014 Equity Incentive Plan | Common Class A | Maximum |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Maximum expiration period of grant date fair value of incentive plan |  |  |  |  | 10 years |  |  |  |
Outstanding Restricted Stock Units |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Grants of restricted stock |  |  |  |  |  |  |  | 970,300 |
Unrecognized compensation cost related to non-vested stock options | $ |  |  |  |  |  | $ 10,108 |  | $ 10,108 |
Weighted-average period for recognition |  |  |  |  |  |  |  | 9 months 29 days |
Outstanding Restricted Stock Units | Officer | Initial Public Offering |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Grants of restricted stock |  | 970,300 |  |  |  |  |  |  |
Non Employee Stock Options |  |  |  |  |  |  |  |  |
Share Based Compensation Arrangement By Share Based Payment Award [Line Items] |  |  |  |  |  |  |  |  |
Shares granted to employee |  |  |  |  |  |  | 0 | 0 |
Option vested |  |  |  |  |  |  |  | 136,933 |
Stock option outstanding |  |  |  |  |  | 347,155 |  | 347,155 |

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