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Stock options and warrants (Schedule of Fair Value Assumptions) (Details) (Stock Incentive Plan [Member])
3 Months Ended
Mar. 31, 2014
Mar. 31, 2013
Stock Incentive Plan [Member]
   
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Dividend yield 0.00% 0.00%
Expected price volatility, minimum 96.00% 127.00%
Expected price volatility, maximum 126.00% 128.00%
Risk free interest rate, minimum 1.52% 0.72%
Risk free interest rate, maximum 1.70% 0.76%
Expected term 5 years 5 years