Collaborative Arrangements and Licensing Agreements, Roche (Details) $ in Thousands	1 Months Ended			3 Months Ended
	Jan. 31, 2018 USD ($)	Dec. 31, 2016 USD ($)	Apr. 30, 2013 USD ($) PerformanceObligation	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)	Jul. 31, 2015 PerformanceObligation
Collaborative Arrangement and Licensing Agreement [Abstract]
Number of separate performance obligations | PerformanceObligation							1
Revenue earned				$ 144,419	$ 115,800
Deferred revenue						$ 233,362
Licensing and Other Royalty Revenue [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Revenue earned				942	2,590
R&D Revenue Under Collaborative Agreements [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Revenue earned				102,396	107,999
Roche [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Upfront payment received			$ 30,000
Milestone payments received		$ 3,000
Maximum amount of payments receivable for license fees and substantive milestones				365,000
Maximum amount of payments receivable for development milestones				70,000
Maximum amount of payments receivable for regulatory milestones				170,000
Maximum amount of payments receivable for commercialization milestones				80,000
Maximum amount of payment receivable for each additional drug developed				136,500
Cumulative payments received				105,000
Next prospective milestone				10,000
Number of separate performance obligations | PerformanceObligation			1
Transaction price			$ 30,000
Revenue earned				2,000	$ (1,600)
Deferred revenue				$ 0		$ 0
Roche [Member] | IONIS-HTT [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Revenue earned	$ 45,000