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Stock-Based Compensation - Significant Assumptions Used to Estimate Fair Value of Employee Stock Purchase Plan (Details) (Employee Stock Purchase Plan)
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3 Months Ended | |
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Mar. 31, 2015
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Mar. 31, 2014
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| Share Based Compensation Arrangement By Share Based Payment Award [Line Items] | ||
| Expected dividends | 0.00% | 0.00% |
| Expected volatility, maximum | 30.73% | 42.11% |
| Risk-free interest rate, maximum | 0.49% | 0.30% |
| Expected volatility, minimum | 30.04% | 32.30% |
| Risk-free interest rate, minimum | 0.07% | 0.07% |
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Maximum
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| Share Based Compensation Arrangement By Share Based Payment Award [Line Items] | ||
| Expected life (years) | 2 years | 2 years |
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Minimum
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| Share Based Compensation Arrangement By Share Based Payment Award [Line Items] | ||
| Expected life (years) | 6 months | 6 months |