Annual report pursuant to Section 13 and 15(d)

Income Taxes - Effective Tax Rate Reconciliation (Details)

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Income Taxes - Effective Tax Rate Reconciliation (Details) - USD ($)
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Operating loss carryforwards    
Interest and penalties related to uncertain tax positions $ 0 $ 0
Research and development credit retroactively reinstated $ 2,400,000 $ 1,800,000
Percent of pre-tax income:    
U.S. federal statutory income tax rate (as a percent) 34.00% 34.00%
Permanent Differences (as a percent) 0.10% (0.60%)
State taxes, net of federal benefit (as a percent) 6.60% 6.50%
Research and development credit (as a percent) 3.90% 3.90%
Other 0.30%  
Change in valuation allowance (as a percent) (44.90%) (43.80%)
Effective income tax rate (as a percent) 0.00% 0.00%
Research and development.    
Operating loss carryforwards    
Uncertain tax position recognized $ 0 $ 0
U.S. federal    
Operating loss carryforwards    
Net operating loss carryforwards 28,700,000 22,800,000
U.S. federal | Research and development.    
Operating loss carryforwards    
Tax credit carryforwards 6,800,000 4,700,000
U.S. state    
Operating loss carryforwards    
Net operating loss carryforwards $ 28,700,000 $ 22,800,000