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Income taxes - Narrative (Details) - USD ($)
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Income Taxes [Line Items]    
Increase (decrease) in valuation allowance $ 9,900,000  
U.S. Federal    
Income Taxes [Line Items]    
Net operating loss carryforward 28,200,000  
Research and development tax credit carryforwards 500,000  
U.S. State    
Income Taxes [Line Items]    
Net operating loss carryforward 24,100,000  
Research and development tax credit carryforwards   $ 200,000
U.S. Federal and State    
Income Taxes [Line Items]    
Operating loss carryforwards that will increase additional paid in capital if used to reduce income taxes payable $ 100,000