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Weighted-Average Assumptions for Stock Option (Detail)
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3 Months Ended | 9 Months Ended | ||
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Sep. 30, 2013
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Sep. 30, 2012
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Sep. 30, 2013
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Sep. 30, 2012
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Stock Options
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| Weighted-average assumptions for stock options and ESPP | ||||
| Risk-free interest rate | 0.70% | 0.62% | 0.80% | 0.62% |
| Expected volatility of common stock | 85.76% | 82.22% | 86.10% | 82.14% |
| Dividend yield | 0.00% | 0.00% | 0.00% | 0.00% |
| Expected term (in years) | 5 years 4 months 24 days | 5 years 3 months 18 days | 5 years 6 months | 5 years 3 months 18 days |
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ESPP
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| Weighted-average assumptions for stock options and ESPP | ||||
| Risk-free interest rate | 0.15% | 0.16% | 0.18% | 0.34% |
| Expected volatility of common stock | 91.60% | 74.34% | 91.55% | 75.52% |
| Dividend yield | 0.00% | 0.00% | 0.00% | 0.00% |
| Expected term (in years) | 6 months | 6 months | 6 months | 6 months |