Accounting policies - Revenue recognized and billed on services (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Accounting policies
Amount of revenue recognized on a subsidiary that was sold		$ 127,000,000
Revenue recognized to date on contracts in progress	$ 8,186,878	17,534,449
Amounts billed to date on contracts in progress	(7,977,023)	(17,791,928)
Retainage	414,011	312,130
Net contract asset	623,866	54,651
Contract assets	669,419	516,521
Contract liabilities	(45,553)	(461,870)
Net contract asset	$ 623,866	$ 54,651