Collaboration agreements - Merck KGaA - Accounting Analysis (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019 USD ($) item	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) item	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($) item
Collaboration agreements
Collaboration revenue	$ 844	$ 2,151	$ 42,081	$ 9,405
Research and development	13,701	15,180	42,610	40,098
Accounts receivable	298		298		$ 459
Deferred revenue	4,646	46,906	4,646	46,906	46,196	$ 52,439
Merck KGaA Agreement and Merck KGaA Supply Agreement
Collaboration agreements
Deferred revenue	$ 4,646		$ 4,646
Merck KGaA
Collaboration agreements
Number of targets designated | item	6		6			6
Estimated transaction price	$ 21,500		$ 21,500		22,070
Decrease in fees expected to be received for research and development activities	570		570
Amount of research and development fees expected to be received	6,500		6,500
Collaboration revenue	800	$ 1,482	836	$ 2,426
Merck KGaA Supply Agreement
Collaboration agreements
Collaboration revenue	34		1,255
Research and development	34		1,255
Accounts receivable	$ 262		$ 262		$ 450
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-01-01 | Merck KGaA | Rights to future technological improvements
Collaboration agreements
Expected recognition period	10 years		10 years
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-01-01 | Merck KGaA | Joint research committee services
Collaboration agreements
Expected recognition period	5 years		5 years