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Income Taxes - Reconciliation of income tax (Details)
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Reconciliation of income tax:      
Income tax (benefit) provision at federal statutory rate (as a percent) (34.00%) (34.00%) (34.00%)
Valuation allowance 40.20% 38.20% 36.80%
State income tax, net of federal benefit (as a percent) (4.70%) (4.00%) (4.00%)
Convertible notes (as a percent) 1.10% 0.60% 1.20%
Research credits (as a percent) (4.00%)    
Other (as a percent) 1.40% (0.80%)  
Effective tax rate (as a percent) 0.00% 0.00% 0.00%