Benefit plans (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019
United States | 401(k) Plan
Benefit plans
Contributions made	$ 26,000	$ 23,000	$ 100,000
Foreign Plan | U.K. Plan
Benefit plans
Contributions made	$ 100,000		$ 200,000
Percentage of employer contribution on annual base salary			12.00%