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 ($)	Sep. 30, 2018 USD ($) item	Mar. 31, 2014 USD ($) item
2014, 2015 and 2016 Takeda Agreements
Collaboration agreements
Number of targets	7	7
Fee payments received | $			$ 24,800
Number of targets designated				4
Number of targets for which Tokeda has limited replacement rights				2
Amount to be received for second limited replacement right | $				$ 500
Aggregate milestones | $				474,750
Development milestones | $				50,250
Regulatory milestones | $				153,000
Commercial milestones | $				$ 271,500
Number of remaining eligible targets				3
Milestone payable on each of the remaining eligible targets | $				$ 158,250
Number of individual development milestones per target				4
Number of individual regulatory milestones per target				8
Number of individual commercial milestones				6
Number of thresholds for annual net sales				3
Next potential development milestone payment eligible to receive | $				$ 750
2015 and 2016 Takeda Agreements
Collaboration agreements
Number of targets	5	5
2014 Tokeda Agreement
Collaboration agreements
Number of targets					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	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	1