Benefit plans (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
United States | 401(k) Plan
Benefit plans
Contributions made	$ 200,000	$ 100,000	$ 42,000
Foreign Plan | U.K. Plan
Benefit plans
Contributions made	$ 300,000	$ 200,000	$ 200,000