Benefit plans (Details) - USD ($)	3 Months Ended	6 Months Ended
Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
United States \| 401(k) Plan
Benefit plans
Contributions made	$ 38,000	$ 25,000	$ 100,000	$ 100,000
Foreign Plan \| U.K. Plan
Benefit plans
Contributions made	$ 100,000	$ 100,000	$ 200,000	$ 200,000

Amount of cost for defined contribution plan.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 70
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=49170846&loc=d3e28014-114942

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration