Commitments and contingencies - Operating Lease, Cash Incentive Bonus Plan, Inducement Option Grant (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Jan. 01, 2019	Dec. 31, 2018	Dec. 31, 2015
Commitments
Operating lease liabilities	$ 4,072,000		$ 4,072,000
Deferred rent balance						$ 989,000
Bonus Expenses	381,000	$ 353,000	1,376,000	$ 1,164,000
Grand Prairie Lease Plus Blue Bell Lease
Commitments
Rent expense, share of operating expenses	53,000	44,000	165,000	150,000
Rent expense, excluding share of operating expenses	$ 253,000	$ 253,000	$ 758,000	$ 758,000
Blue Bell, Pennsylvania
Commitments
Term of operating lease							60 months
ASU 2016-02
Commitments
Operating lease liabilities					$ 4,300,000