Summary of significant accounting policies - Concentration of customers and concentration of suppliers (Details) - USD ($) $ in Thousands	6 Months Ended
Jun. 30, 2020	Jun. 30, 2019
Summary of significant accounting policies
Revenue	$ 19,213	$ 3,420
Research and development expenses	102,049	58,928
Revenue \| Concentration of customers \| A
Summary of significant accounting policies
Revenue		920
Revenue \| Concentration of customers \| D
Summary of significant accounting policies
Revenue		1,102
Revenue \| Concentration of customers \| E
Summary of significant accounting policies
Revenue	3,236
Research and Development Expenses \| Concentration of suppliers \| B
Summary of significant accounting policies
Research and development expenses	12,313
Research and Development Expenses \| Concentration of suppliers \| C
Summary of significant accounting policies
Research and development expenses		$ 22,625
Research and Development Expenses \| Concentration of suppliers \| E
Summary of significant accounting policies
Research and development expenses	$ 31,022

The aggregate costs incurred (1) in a planned search or critical investigation aimed at discovery of new knowledge with the hope that such knowledge will be useful in developing a new product or service, a new process or technique, or in bringing about a significant improvement to an existing product or process; or (2) to translate research findings or other knowledge into a plan or design for a new product or process or for a significant improvement to an existing product or process whether intended for sale or the entity's use, during the reporting period charged to research and development projects, including the costs of developing computer software up to the point in time of achieving technological feasibility, and costs allocated in accounting for a business combination to in-process projects deemed to have no alternative future use.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 985
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6501960&loc=d3e128462-111756

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 730
-SubTopic 10
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6420194&loc=d3e21568-108373

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