Summary of significant accounting policies - Cash, Cash Equivalents, Restricted Cash, and Marketable Securities (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Accounting Policies [Abstract]
Restricted cash	$ 758,000	$ 758,000
Minimum Maturity for Short Term Investments	3 months
Maximum Maturity for Cash and Cash Equivalent at Date of Purchase	3 months