Income taxes - Deferred tax assets and liabilities (Details) - USD ($)	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Deferred tax assets:
Net operating loss carryforwards	$ 22,842,817	$ 25,425,715
Stock options	3,458,886	3,482,800
Bonuses and salaries	26,988	26,993
Restricted stock	247,620	91,580
Provision for doubtful accounts	10,987	14,568
Foreign nonrefundable withholding tax		478,295
Foreign tax credit	589,188	335,698
Total deferred tax assets	27,176,486	29,855,649
Less: Valuation allowance	(20,561,516)	(28,475,080)	$ (29,400,000)
Net deferred tax assets	6,614,970	1,380,569
Deferred tax liabilities:
Depreciation and amortization	(408,657)	(653,074)
Total deferred tax liabilities	(408,657)	(653,074)	(1,000,000)
Total deferred tax assets, net	$ 6,206,313	$ 727,495	$ 800,000