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Collaboration agreements - Merck KGaA - Accounting Analysis (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2020 USD ($) item	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 item
Collaboration agreements
Collaboration revenue	$ 11	$ 41,035
Research and development	12,219	15,143
Deferred revenue	4,804	5,604	$ 4,815	$ 46,196
Merck KGaA Agreement and Merck KGaA Supply Agreement
Collaboration agreements
Deferred revenue	$ 4,804		$ 4,815
Merck KGaA
Collaboration agreements
Number of targets designated | item	6				6
Collaboration revenue	$ 11	18
Merck KGaA | Rights to future technological improvements | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-01-01
Collaboration agreements
Expected recognition period	10 years
Merck KGaA | Joint research committee services | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-01-01
Collaboration agreements
Expected recognition period	5 years
Merck KGaA Supply Agreement
Collaboration agreements
Collaboration revenue	$ 0	1,037
Research and development	$ 0	$ 1,037

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Amount of obligation to transfer good or service to customer for which consideration has been received or is receivable.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 45
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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.

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