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Accounting policies - Revenue recognized and billed on services (Details) - USD ($)
 3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Dec. 31, 2020
Accounting policies        
Amount of revenue recognized on a subsidiary that was sold $ 46,000   $ 118,000  
Revenue recognized to date on contracts in progress   $ 6,808,572   $ 17,534,449
Amounts billed to date on contracts in progress   (6,565,955)   (17,791,928)
Retainage   195,341   312,130
Net contract asset   437,958   54,651
Contract assets   483,190   516,521
Contract liabilities   (45,232)   (461,870)
Net contract asset   $ 437,958   $ 54,651