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Warrants, stock-based compensation, stock options, restricted stock and restricted stock units - Restricted stock (Details) - Restricted stock - shares
1 Months Ended 3 Months Ended
Jan. 31, 2015
Dec. 31, 2014
Mar. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Granted (in shares)     0
2007 Plan | Director      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Granted (in shares)   78,912  
Vesting period (in years)   4 years  
2007 Plan | Executive      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Granted (in shares) 781,060    
Vesting period (in years) 4 years    
2007 Plan | Executive | Subject to a four year vesting schedule with 25% vesting on the first anniversary and the remaining vesting ratably on a monthly basis over the remaining three years      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Vesting percentage 1 (as a percent) 25.00%    
2007 Plan | Executive | Subject to vesting with 50% vesting on December 31, 2018 and December 31, 2019      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Vesting period (in years) 3 years    
Vesting percentage 1 (as a percent) 75.00%