Accounting policies - Cash and cash equivalents (Details) - USD ($)	Dec. 31, 2020	Dec. 31, 2019
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash Equivalents, at Carrying Value	$ 9,275,893	$ 13,362,649
Cash, Uninsured Amount	10,300,000
Cash And Restricted Cash Equivalents Held In Foreign Bank	32,800,000
Bahamas [Member]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Deposits held in foreign bank	15,900,000
Certificates of Deposit [Member]
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash Equivalents, at Carrying Value	$ 8,500,000	$ 12,700,000