Income tax (Narrative) (Details) - USD ($)	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Sep. 30, 2013
Income Tax [Line Items]
PRC statutory income tax rate	25.00%
Effective income tax rate	24.90%	24.50%
Change in Effective income tax rate	24.90%	24.50%
Unrecognized tax benefits	$ 34,626,289		$ 3,752,714
Interest relating to unrecognized tax benefits	$ 13,710,137		$ 3,164,360
Percentage of additional deduction of major PRC operating entities	75.00%
Subsidiary One [Member]
Income Tax [Line Items]
PRC statutory income tax rate	15.00%