Income taxes - Provision for income taxes (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Current:
Federal	$ 191,322	$ 143,567
State	87,630	171,093
Total	278,952	314,660
Deferred:
Federal	(132,957)	(203,031)
State	(59,271)	(45,008)
Total	(192,228)	(248,039)
Total provision	$ 86,724	$ 66,621