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Commitments and contingencies (Details Textual) - USD ($)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Commitments and Contingencies [Line Items]    
Estimated Litigation Liability $ 6,200,000  
Litigation Settlement, Expense 100,000  
Operating Leases, Rent Expense $ 2,700,000 $ 2,700,000
Interest Rate Description Litigation plus legal interest that accrued at 12% per annum from August 13, 2008 to June 30, 2013, and thereafter accrued and continues to accrue at 6% per annum.