Summary of significant accounting policies - Reconciliation of cash, cash equivalents and restricted cash (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounting Policies [Abstract]
Cash and cash equivalents	$ 9,829,624	$ 19,186,036
Restricted cash, current	126,595	0
Restricted cash	0	126,595
Cash, cash equivalents and restricted cash shown on the condensed consolidated statement of cash flows	$ 9,956,219	$ 19,312,631	$ 22,542,635	$ 66,813,530

Amount of cash and cash equivalents restricted as to withdrawal or usage, classified as noncurrent. Cash includes, but is not limited to, currency on hand, demand deposits with banks or financial institutions, and other accounts with general characteristics of demand deposits. Cash equivalents include, but are not limited to, short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant