Collaborative Arrangements and Licensing Agreements, Janssen Biotech, Inc. (Details) $ in Thousands	1 Months Ended			3 Months Ended		17 Months Ended
	Nov. 30, 2017 USD ($)	Jul. 31, 2016 USD ($)	Dec. 31, 2014 USD ($) PerformanceObligation Program	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)	Nov. 30, 2017 USD ($)	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
Janssen Biotech, Inc. [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Number of programs under which drugs are to be developed and commercialized | Program			3
Upfront payment received			$ 35,000
Minimum amount of payments receivable for license fees and substantive milestones				800,000
Maximum amount of payments receivable for development milestones				175,000
Maximum amount of payments receivable for regulatory milestones				440,000
Maximum amount of payments receivable for commercialization milestones				180,000
Cumulative payments received				72,000
Next prospective milestone				5,000
Number of separate performance obligations | PerformanceObligation			1
Transaction price			$ 35,000
Deferred revenue				2,800			$ 0
Janssen Biotech, Inc. [Member] | Licensing and Other Royalty Revenue [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Revenue earned						$ 15,000
Janssen Biotech, Inc. [Member] | IONIS-JBI1-2.5 [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Revenue earned		$ 10,000
Janssen Biotech, Inc. [Member] | IONIS-JBI2-2.5 [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Revenue earned	$ 5,000
Janssen Biotech, Inc. [Member] | R&D Revenue Under Collaborative Agreements [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Revenue earned				$ 100	$ 2,500