Commitments and contingencies - Operating Lease, Cash Incentive Bonus Plan, Inducement Option Grant (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2019	Dec. 31, 2015
Commitments
Total lease obligations	$ 3,935,000
Deferred rent balance		$ 989,000
Bonus Expenses	1,567,000	666,000	$ 701,000
Grand Prairie Lease Plus Blue Bell Lease
Commitments
Rent expense, share of operating expenses	217,000	206,000	243,000
Rent expense, excluding share of operating expenses		$ 1,011,000	$ 1,010,000
Rent expense, excluding share of operating expenses	$ 1,011,000
Blue Bell, Pennsylvania
Commitments
Term of operating lease					60 months
ASU 2016-02
Commitments
Total lease obligations				$ 4,300,000
Deferred rent balance				$ 900,000