Benefit plans (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
United States | 401(k) Plan
Benefit plans
Contributions made	$ 25,000	$ 19,000	$ 100,000	$ 100,000
Foreign Plan | U.K. Plan
Benefit plans
Contributions made	$ 100,000	$ 100,000	$ 200,000	$ 100,000
Percentage of employer contribution on annual base salary			12.00%