Collaboration arrangements and concentration of credit risk - BMS collaboration (Details) $ in Thousands		1 Months Ended	3 Months Ended
	May 21, 2015 USD ($)	Aug. 31, 2015 USD ($)	Mar. 31, 2018 USD ($) item	Mar. 31, 2017 USD ($)
Collaboration arrangements
Revenue			$ 3,478
BMS
Collaboration arrangements
Revenue			3,478
License revenues from related party
Collaboration arrangements
Revenue			2,500
Collaborative revenues from related party
Collaboration arrangements
Revenue			$ 1,000
License revenue | BMS
Collaboration arrangements
Number of potential targets included in collaborative agreement | item			10
Upfront payment recorded	$ 60,100
Royalty term after the first commercial sale			10 years
Average period for completion of pre-clinical phase			4 years
Average period for completion of clinical development and commercial launch			8 years 6 months
Percentage of increase in probability of all additional targets			10
License revenue | BMS | Minimum
Collaboration arrangements
Decrease in revenue			$ 700
License revenue | BMS | Maximum
Collaboration arrangements
Decrease in revenue			1,800
License revenue | BMS | First Target Selection
Collaboration arrangements
Milestone payments to be received upon achievement			254,000
License revenue | BMS | Second, Third, and Fourth Targets Selection
Collaboration arrangements
Target designation payment received		$ 15,000
License revenue | BMS | Fifth through Tenth Targets Selection
Collaboration arrangements
Maximum target designation payments to which entitled per agreement			16,500
License revenue | BMS | Other Selected Targets
Collaboration arrangements
Milestone payments to be received upon achievement			$ 217,000
Before Topic 606
Collaboration arrangements
Revenue				$ 3,321
Before Topic 606 | BMS
Collaboration arrangements
Revenue				1,507
Before Topic 606 | License revenues from related party
Collaboration arrangements
Revenue				900
Before Topic 606 | Collaborative revenues from related party
Collaboration arrangements
Revenue				$ 600