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Stock Based Compensation - Stock Option Activity Under Equity Incentive Plan (Detail) (USD $)
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3 Months Ended |
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Mar. 31, 2015
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| Disclosure Of Compensation Related Costs Sharebased Payments [Abstract] |
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| Number of Shares, Outstanding Beginning Balance |
8,168,942 |
| Number of Shares, Granted |
1,911,500 |
| Number of Shares, Exercised |
(61,456) |
| Number of Shares, Forfeitures |
(25,770) |
| Number of Shares, Outstanding Ending Balance |
9,993,216 |
| Number of Shares, Vested and expected to vest |
9,878,829 |
| Number of Shares, Exercisable |
5,521,561 |
| Weighted-Average Exercise Price, Outstanding at beginning of period |
$ 3.69 |
| Weighted-Average Exercise Price, Granted |
$ 4.42 |
| Weighted-Average Exercise Price, Exercised |
$ 1.83 |
| Weighted-Average Exercise Price, Forfeitures |
$ 5.15 |
| Weighted-Average Exercise Price, Outstanding at end of period |
$ 3.83 |
| Weighted-Average Exercise Price, Vested and expected to vest |
$ 3.83 |
| Weighted-Average Exercise Price, Exercisable |
$ 3.33 |
| Weighted-Average Remaining Contractual Term, Outstanding |
7 years 6 months 11 days |
| Weighted-Average Remaining Contractual Term, Vested and expected to vest |
7 years 6 months 4 days |
| Weighted-Average Remaining Contractual Term, Exercisable |
6 years 3 months 22 days |
| Aggregate Intrinsic Value, Outstanding |
$ 8,802,913 |
| Aggregate Intrinsic Value, Vested and expected to vest |
8,787,468 |
| Aggregate Intrinsic Value, Exercisable |
$ 8,010,861 |