Income taxes - Deferred tax assets and liabilities (Details) - USD ($)	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Deferred tax assets:
Net operating loss carryforwards	$ 21,030,069	$ 22,842,817
Stock options	1,580,016	3,458,886
Bonuses and salaries	22,825	26,988
Restricted stock	151,358	247,620
Provision for doubtful accounts	46,607	10,987
Foreign tax credit	1,028,169	589,188
Total deferred tax assets	23,859,044	27,176,486
Less: Valuation allowance	(928,701)	(20,561,516)	$ (28,500,000)
Net deferred tax assets	22,930,343	6,614,970
Deferred tax liabilities:
Depreciation and amortization	(257,137)	(408,657)
Total deferred tax liabilities	(257,137)	(408,657)	(700,000)
Total deferred tax assets, net	$ 22,673,206	$ 6,206,313	$ 700,000