Commitments and contingencies - Operating Lease, Cash Incentive Bonus Plan, Inducement Option Grant (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Future minimum operating lease payments
2019	$ 1,107,000
2020	1,161,000
2021	1,057,000
2022	1,055,000
2023	1,055,000
Thereafter	1,106,000
Future minimum lease payments	6,541,000
Deferred rent balance	989,000	$ 1,083,000
Rent expense, share of operating expenses	206,000	243,000	$ 243,000
Bonus Expenses	666,000	701,000	464,000
Grand Prairie Lease Plus Blue Bell Lease
Future minimum operating lease payments
Rent expense, excluding share of operating expenses	$ 1,011,000	$ 1,010,000	$ 1,011,000
Blue Bell, Pennsylvania
Commitments
Term of operating lease	60 months