Benefit plans (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
United States | 401(k) Plan
Benefit plans
Contributions made	$ 100,000	$ 34,000	$ 200,000	$ 100,000
Foreign Plan | U.K. Plan
Benefit plans
Contributions made	$ 200,000	$ 100,000	$ 500,000	$ 300,000