Collaboration agreements - Takeda partnership (Details) $ in Thousands		1 Months Ended	49 Months Ended
	Jan. 31, 2016 USD ($) item	Jan. 31, 2016 USD ($) item	Mar. 31, 2018 USD ($) item	Mar. 31, 2014 USD ($) item
2014, 2015 and 2016 Takeda Agreements
Collaboration agreements
Number of targets | item	7	7
Fee payments received | $			$ 24,800
Number of targets designated | item			4
Number of targets for which Tokeda has limited replacement rights | item			2
Amount to be received for second limited replacement right | $			$ 500
Aggregate milestones | $			769,000
Development milestones | $			79,000
Regulatory milestones | $			239,000
Commercial milestones | $			451,000
Milestone on first designated target | $			136,000
Milestone on each of the third, fifth sixth and seventh designated targets | $			$ 158,250
Number of individual development milestones per target | item			4
Number of individual commercial milestones | item			6
Number of thresholds for annual net sales | item			3
Next potential development milestone payment eligible to receive | $			$ 500
Amount of next potential milestone receivable related to filing IND | $			$ 750
2014, 2015 and 2016 Takeda Agreements | Minimum
Collaboration agreements
Number of individual regulatory milestones per target | item			6
2014, 2015 and 2016 Takeda Agreements | Maximum
Collaboration agreements
Number of individual regulatory milestones per target | item			8
2015 and 2016 Takeda Agreements
Collaboration agreements
Number of targets | item	5	5
2014 Tokeda Agreement
Collaboration agreements
Number of targets | item				2
Amount of option exercise fee for designated targets | $				$ 1,300
2016 Restated Tokeda Agreement
Collaboration agreements
Number of products entity may elect to co-develop and co-commercialize | item	1	1
Payment for option to co-develop and co-commercialize product | $	$ 15,000	$ 15,000
Share of profit related to United States (as a percent)	50.00%	50.00%
Costs incurred in United States (as a percent)	50.00%	50.00%
Global development costs (as a percent)	30.00%	30.00%
Costs incurred specifically for a foreign country borne by Takeda (as a percent)	100.00%	100.00%
Prior written notice for termination period		45 days
Number of targets with respect to breach | item	1