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Employee Benefit Plans, Assumptions and Resulting Estimates of Fair Value (Details) (USD $)
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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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| Estimates of Fair Value | ||||
| Volatility | 46.00% | 46.00% | ||
| Risk-free interest rate | 0.70% | 0.80% | ||
| Expected life (years) | 4 years 2 months 12 days | 4 years 2 months 12 days | ||
| Dividend yield | 0.00% | 0.00% | ||
| Weighted-average fair value per share | $ 6.89 | $ 8.46 | ||
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Employee stock purchase plan
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| Estimates of Fair Value | ||||
| Volatility | 35.00% | 42.00% | 36.00% | 47.00% |
| Risk-free interest rate | 0.10% | 0.10% | 0.10% | 0.10% |
| Expected life (years) | 6 months | 6 months | 6 months | 6 months |
| Dividend yield | 0.00% | 0.00% | 0.00% | 0.00% |
| Weighted-average fair value per share | $ 5.47 | $ 4.77 | $ 5.54 | $ 5.53 |