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Income taxes - Additional Information (Details) - USD ($)
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Income Tax Disclosure [Abstract]      
State net operating loss carryforwards $ 600,000    
Net operating loss carryforwards period 20 years    
Unrecognized tax benefits that would impact effective tax rate $ 400,000 $ 300,000  
Unrecognized tax benefits expected to reverse in next twelve months 200,000    
Accrued gross interest and penalties 100,000 100,000  
Recognized net interest expense $ (40,000) $ 17,000 $ 100,000