Accounting policies - Revenue recognized and billed on services (Details) - USD ($)	6 Months Ended	12 Months Ended
	Jun. 30, 2020	Dec. 31, 2019
Accounting policies
Revenues recognized to date on contracts in progress	$ 23,805,174	$ 24,041,993
Amounts billed to date on contracts in progress	(22,502,592)	(22,980,598)
Total revenue recognized and amount billed on contracts in progress	1,302,582	1,061,395
Costs and estimated earnings in excess of billings	2,202,524	1,675,781
Billings in excess of costs and estimated earnings	(899,942)	(614,386)
Costs and estimated earnings and billings in excess of costs	$ 1,302,582	$ 1,061,395