Revenue from contracts with customers (Details) - USD ($)	3 Months Ended			6 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2018	Sep. 30, 2017	Jun. 18, 2018	Sep. 30, 2018	Sep. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Increase to retained earnings due to adoption of Topic 606	$ (130,952,530)	$ (130,952,530)			$ (130,952,530)			$ (111,079,275)
Total revenue		250,991	$ 413,601		675,075	$ 991,671
Refund period	14 days			30 days
Discounts to customers		2,000	$ 82,000		19,000	$ 202,000
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Increase to retained earnings due to adoption of Topic 606							$ 40,000
Decrease in deferred revenue							70,000
Decrease in deferred costs from product revenue							$ 30,000
Total revenue		$ (1,000)			$ (29,000)