Collaboration agreements - Takeda partnership (Details) $ in Thousands	1 Months Ended			9 Months Ended	12 Months Ended
	Jan. 31, 2016 USD ($) product item	Jan. 31, 2015 USD ($) item	Mar. 31, 2014 USD ($) item	Dec. 31, 2014 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2017 USD ($) item
2014, 2015 and 2016 Takeda Agreements
Collaboration agreements
Number of targets | item	7
Number of targets designated | item						4
Amount of option exercise fee for second designated target						$ 1,300
Number of targets Tokeda has remaining | item						3
Number of targets Tokeda has the limited right to replace | item						2
Aggregate milestones						$ 1,063,300
Development milestones						107,800
Regulatory milestones						325,000
Commercial milestones						630,500
Milestone on each of the first and second designated targets						136,000
Milestone on each of the remaining designated targets						$ 158,300
Number of individual development milestones per target | item						4
Number of individual commercial milestones per target | 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
2014 Tokeda Agreement
Collaboration agreements
Number of targets | item			2
Amount of technology access fee per target			$ 500
Amount of option exercise fee for first designated target			1,300
Upfront payment received			1,150
Technology access fee for first designated target received			500
Option exercise fee for first designated target received			650	$ 650
Amount of option exercise fee for first designated target			$ 1,300
Technology access fee for second designated target received				$ 500	$ 500
2015 Amended Tokeda Agreement
Collaboration agreements
Number of targets | item		2
Upfront payment received		$ 9,000
Amount to be received for second limited replacement right		$ 500
2016 Restated Tokeda Agreement
Collaboration agreements
Number of targets | item	3
Upfront payment received	$ 13,500
Number of products entity may elect to co-develop and co-commercialize | product	1
Payment for option to co-develop and co-commercialize product	$ 15,000
Share of profit related to United States (as a percent)	50.00%
Costs incurred in United States (as a percent)	50.00%
Global development costs (as a percent)	30.00%
Costs incurred specifically for a foreign country borne by Takeda (as a percent)	100.00%
Prior written notice for termination period	45 days