Segment Information and Concentration of Business Risk, Segment Information (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2019 USD ($) Segment	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jun. 30, 2017
Segment Information [Abstract]
Number of reportable segments | Segment									2
Segment Information [Abstract]
Revenue									$ 1,122,599	$ 599,674	$ 514,179
Total operating expenses	$ 233,028	$ 165,369	$ 182,640	$ 175,679	$ 181,331	$ 163,967	$ 168,028	$ 147,720	756,716	661,046	483,132
Income (loss) from operations	260,652	$ 2,523	$ (18,827)	$ 121,535	10,782	$ (18,572)	$ (50,281)	$ (3,301)	365,883	(61,372)	31,047
Total assets as of current period	$ 3,233,112				2,667,784				3,233,112	2,667,784
Commercial Revenue [Member]
Segment Information [Abstract]
Revenue									352,450	254,922	120,014
SPINRAZA Royalties [Member]
Segment Information [Abstract]
Revenue									292,992	237,930	112,540
Product Sales, Net [Member]
Segment Information [Abstract]
Revenue									42,253	2,237	0
Licensing and Other Royalty Revenue [Member]
Segment Information [Abstract]
Revenue									17,205	14,755	7,474
R&D Revenue Under Collaborative Agreements [Member]
Segment Information [Abstract]
Revenue									$ 770,149	344,752	394,165
Akcea [Member]
Segment Information [Abstract]
Percentage ownership	76.00%								76.00%			100.00%
Operating Segments [Member] | Ionis Core [Member]
Segment Information [Abstract]
Revenue									$ 858,646	641,944	525,185
Total operating expenses									523,207	380,212	373,788
Income (loss) from operations									335,439	261,732	151,397
Total assets as of current period	$ 3,478,081				2,975,491				3,478,081	2,975,491
Operating Segments [Member] | Ionis Core [Member] | Commercial Revenue [Member]
Segment Information [Abstract]
Revenue									305,608	240,685	120,014
Operating Segments [Member] | Ionis Core [Member] | SPINRAZA Royalties [Member]
Segment Information [Abstract]
Revenue									292,992	237,930	112,540
Operating Segments [Member] | Ionis Core [Member] | Product Sales, Net [Member]
Segment Information [Abstract]
Revenue									0	0
Operating Segments [Member] | Ionis Core [Member] | Licensing and Other Royalty Revenue [Member]
Segment Information [Abstract]
Revenue									12,616	2,755	7,474
Operating Segments [Member] | Ionis Core [Member] | R&D Revenue Under Collaborative Agreements [Member]
Segment Information [Abstract]
Revenue									553,038	401,259	405,171
Operating Segments [Member] | Akcea Therapeutics [Member]
Segment Information [Abstract]
Revenue									488,543	64,867	43,401
Total operating expenses									450,469	295,683	163,871
Income (loss) from operations									38,074	(230,816)	(120,470)
Total assets as of current period	599,250				365,261				599,250	365,261
Operating Segments [Member] | Akcea Therapeutics [Member] | Commercial Revenue [Member]
Segment Information [Abstract]
Revenue									52,425	14,237	0
Operating Segments [Member] | Akcea Therapeutics [Member] | SPINRAZA Royalties [Member]
Segment Information [Abstract]
Revenue									0	0	0
Operating Segments [Member] | Akcea Therapeutics [Member] | Product Sales, Net [Member]
Segment Information [Abstract]
Revenue									42,253	2,237
Operating Segments [Member] | Akcea Therapeutics [Member] | Licensing and Other Royalty Revenue [Member]
Segment Information [Abstract]
Revenue									10,172	12,000	0
Operating Segments [Member] | Akcea Therapeutics [Member] | R&D Revenue Under Collaborative Agreements [Member]
Segment Information [Abstract]
Revenue									436,118	50,630	43,401
Elimination of Intercompany Activity [Member]
Segment Information [Abstract]
Revenue									(224,590)	(107,137)	(54,407)
Total operating expenses									(216,960)	(14,849)	(54,527)
Income (loss) from operations									(7,630)	(92,288)	120
Total assets as of current period	$ (844,219)				$ (672,968)				(844,219)	(672,968)
Elimination of Intercompany Activity [Member] | Commercial Revenue [Member]
Segment Information [Abstract]
Revenue									(5,583)	0	0
Elimination of Intercompany Activity [Member] | SPINRAZA Royalties [Member]
Segment Information [Abstract]
Revenue									0	0	0
Elimination of Intercompany Activity [Member] | Product Sales, Net [Member]
Segment Information [Abstract]
Revenue									0	0
Elimination of Intercompany Activity [Member] | Licensing and Other Royalty Revenue [Member]
Segment Information [Abstract]
Revenue									(5,583)	0	0
Elimination of Intercompany Activity [Member] | R&D Revenue Under Collaborative Agreements [Member]
Segment Information [Abstract]
Revenue									$ (219,007)	$ (107,137)	$ (54,407)