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Income Taxes - Rate Reconciliation (Details)
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12 Months Ended | ||
|---|---|---|---|
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Dec. 31, 2013
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Dec. 31, 2012
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Dec. 31, 2011
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| Income Tax Disclosure [Abstract] | |||
| U.S. Federal statutory rate | 35.00% | 35.00% | 35.00% |
| State taxes | 5.00% | 5.00% | 5.10% |
| Research credits | 13.30% | 0.00% | 3.40% |
| Stock-based compensation | (0.50%) | (0.50%) | (0.40%) |
| Other | (0.10%) | (0.20%) | (0.10%) |
| Valuation allowance | (52.70%) | (39.30%) | (43.00%) |
| Effective Income Tax Rate | 0.00% | 0.00% | 0.00% |