Income taxes - Components of income before income taxes (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Income taxes
Foreign (not subject to income taxes)	$ 6,475,693	$ 13,593,497
Mexico	(4,903,988)	(2,458,210)
United States	2,956,552	2,657,405
Income before income taxes	4,528,257	13,792,692
Less gain on sale of discontinued operations	0	(3,621,170)
Less discontinued operations		2,333,255
Income before income taxes	$ 9,430,500	$ 12,504,777