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Cash and cash equivalents (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash and Cash Equivalents [Line Items]
Bank accounts	$ 30,794,936	$ 37,754,409
Short term deposits	16,388,030	1,499,707
Total cash and cash equivalents	47,182,966	39,254,116	$ 44,792,734	$ 35,713,689
United States dollar [Member]
Cash and Cash Equivalents [Line Items]
Bank accounts	6,764,201	11,350,895
Short term deposits	10,559,407	490,914
Cayman Islands dollar [Member]
Cash and Cash Equivalents [Line Items]
Bank accounts	4,306,768	16,511,673
Short term deposits	4,802,060	0
Bahamian dollar [Member]
Cash and Cash Equivalents [Line Items]
Bank accounts	13,310,936	4,859,914
Short term deposits	1,026,563	1,008,793
Belize dollar [Member]
Cash and Cash Equivalents [Line Items]
Bank accounts	6,346,184	4,884,161
Bermudian dollar [Member]
Cash and Cash Equivalents [Line Items]
Bank accounts	3,502	3,733
Mexican peso [Member]
Cash and Cash Equivalents [Line Items]
Bank accounts	17,014	83,752
Euro Member Countries, Euro
Cash and Cash Equivalents [Line Items]
Bank accounts	0	5,221
Singapore dollar [Member]
Cash and Cash Equivalents [Line Items]
Bank accounts	0	25,840
Indonesian rupiah [Member]
Cash and Cash Equivalents [Line Items]
Bank accounts	$ 46,331	$ 29,220