Collaborative Arrangements and Licensing Agreements, Biogen (Details) $ in Thousands	1 Months Ended				2 Months Ended	3 Months Ended							12 Months Ended
	Dec. 31, 2019 USD ($) Drug Country	Dec. 31, 2017 USD ($) PerformanceObligation	Sep. 30, 2013 USD ($) PerformanceObligation	Dec. 31, 2012 USD ($) Program Target	Feb. 26, 2020 USD ($)	Dec. 31, 2019 USD ($) Drug Country Target	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Sep. 30, 2017 Payment	Dec. 31, 2019 USD ($) Drug Payment Target Country	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($) PerformanceObligation	Apr. 30, 2018 USD ($) PerformanceObligation
Collaborative Arrangement and Licensing Agreement [Abstract]
Revenue													$ 1,122,599	$ 599,674	$ 514,179
SPINRAZA Royalties [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Revenue													292,992	237,930	112,540
Licensing and Other Royalties [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Revenue													17,205	14,755	7,474
R&D Revenue [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Revenue													$ 770,149	344,752	394,165
Biogen [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Number of drugs currently being developed | Drug	8					8							8
Cumulative payments received	$ 2,400,000					$ 2,400,000							$ 2,400,000
Revenue													473,600	375,000	$ 263,100
Deferred revenue	$ 525,800					$ 525,800			$ 580,900				$ 525,800	$ 580,900
Biogen [Member] | Revenue [Member] | Strategic Partner [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Concentration percentage													42.00%	63.00%	51.00%
Biogen [Member] | SPINRAZA Royalties [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Revenue													$ 293,000	$ 237,900	$ 112,500
Biogen [Member] | R&D Revenue [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Revenue													$ 180,600	$ 137,100	150,600
SPINRAZA [Member] | Minimum [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Number of countries where SPINRAZA is approved for use | Country	50					50							50
Cumulative revenue earned	$ 1,000,000					$ 1,000,000							$ 1,000,000
Royalty percentage received on net sales of drug													11.00%
SPINRAZA [Member] | Maximum [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Royalty percentage received on net sales of drug													15.00%
SPINRAZA [Member] | SPINRAZA Royalties [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Cumulative revenue earned	640,000					640,000							$ 640,000
SPINRAZA [Member] | R&D Revenue [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Cumulative revenue earned	435,000					435,000							$ 435,000
Revenue															$ 90,000
New Antisense Medicines for the Treatment of SMA [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Royalty percentage received on net sales of drug													20.00%
Upfront payment received		$ 25,000
Maximum amount of payments receivable for license fees and milestones	1,200,000					1,200,000							$ 1,200,000
Maximum amount of payments receivable for development milestones	80,000					80,000							80,000
Maximum amount of payments receivable for commercialization milestones	180,000					180,000							180,000
Maximum amount of payments receivable for sales milestones	800,000					800,000							800,000
Next prospective payment	60,000					60,000							60,000
Number of separate performance obligations | PerformanceObligation		1													1
Transaction price		$ 25,000													$ 25,000
Revenue						8,300
2018 Strategic Neurology [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Cumulative payments received	1,000,000					1,000,000							$ 1,000,000
Upfront payment received, including purchase of stock											$ 1,000,000
Royalty percentage received on net sales of drug													20.00%
Upfront payment received											375,000
Next prospective payment	7,500					7,500							$ 7,500
Number of separate performance obligations | PerformanceObligation																1
Transaction price																$ 552,000
Term of collaboration agreement													10 years
Proceeds from issuance of common stock											$ 625,000
Percentage cash premium paid on shares purchased											25.00%
Maximum amount of payments receivable per drug for substantive milestone payments	270,000					$ 270,000							$ 270,000
Number of targets advanced | Target						2							4
Premium paid on shares purchased											$ 177,000
Number of milestone payments achieved | Payment												2	4
Cumulative payments included in transaction price for performance obligation	597,000					$ 597,000							$ 597,000
Payments included in transaction price for performance obligation													7,500
Milestone payment achieved									15,000
2013 Strategic Neurology [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Cumulative payments received	240,000					240,000							240,000
Upfront payment received			$ 100,000
Next prospective payment	$ 10,000					$ 10,000							$ 10,000
Number of separate performance obligations | PerformanceObligation			1
Transaction price			$ 100,000
Number of drugs currently being advanced | Drug	5					5							5
Revenue							$ (16,500)
2013 Strategic Neurology [Member] | Licensing and Other Royalties [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Revenue									35,000
2013 Strategic Neurology [Member] | R&D Revenue [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Cumulative payments included in transaction price for performance obligation	$ 145,000					$ 145,000							$ 145,000
2013 Strategic Neurology [Member] | Medicines for ALS [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Number of drugs currently being advanced | Drug	2					2							2
Number of drugs licensed under option | Drug	1
2013 Strategic Neurology [Member] | Medicines for Undisclosed Targets [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Number of drugs currently being advanced | Drug	2					2							2
2013 Strategic Neurology [Member] | Antisense Molecule [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Maximum amount of payments receivable per program for license fee and milestone payments	$ 260,000					$ 260,000							$ 260,000
Maximum amount of payments receivable per program for development milestones	60,000					60,000							60,000
Maximum amount of payments receivable per program for regulatory milestones	130,000					130,000							130,000
2013 Strategic Neurology [Member] | IONIS-C9 [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Revenue						10,000				$ 10,000
2013 Strategic Neurology [Member] | Tofersen [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Revenue									$ 5,000
2013 Strategic Neurology [Member] | ION859 [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Revenue							8,000
2012 Neurology [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Cumulative payments received	130,000					130,000							130,000
Upfront payment received				$ 30,000
Next prospective payment	12,000					12,000							12,000
Maximum amount of payments receivable per program for license fee and milestone payments	210,000					210,000							210,000
Maximum amount of payments receivable per program for development milestones	10,000					10,000							10,000
Maximum amount of payments receivable per program for regulatory milestones	130,000					130,000							130,000
Number of drugs to be developed and commercialized | Target				3
Number of programs under which drugs are to be developed and commercialized | Program				3
Revenue						10,000	$ 6,300
2012 Neurology [Member] | Subsequent Event [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Revenue					$ 7,500
2012 Neurology [Member] | Licensing and Other Royalties [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Revenue						45,000
2012 Neurology [Member] | R&D Revenue [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Cumulative payments included in transaction price for performance obligation	40,000					40,000							40,000
2012 Neurology [Member] | IONIS-MAPT [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Cumulative payments included in transaction price for performance obligation	$ 37,500					37,500							$ 37,500
Milestone payment achieved						12,000		$ 7,500
Revenue						45,000
2012 Neurology [Member] | ION581 [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Revenue						$ 10,000