Benefit plans (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
United States | 401(k) Plan
Benefit plans
Contributions made	$ 48,000	$ 38,000	$ 100,000	$ 100,000
Foreign Plan | U.K. Plan
Benefit plans
Contributions made	$ 200,000	$ 100,000	$ 300,000	$ 200,000