Segment Information and Concentration of Business Risk, Segment Information (Details) $ in Thousands	3 Months Ended		12 Months Ended
	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2020 USD ($) Segment	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Segment Information and Concentration of Business Risk [Abstract]
Number of reportable segments | Segment			2
Segment Information [Abstract]
Revenue			$ 729,264	$ 1,122,599	$ 599,674
Total operating expenses	$ 312,945	$ 233,028	901,346	756,716	661,046
Income (loss) from operations	(22,664)	260,652	(172,082)	365,883	(61,372)
Total assets	2,389,755	3,233,112	2,389,755	3,233,112
Commercial Revenue [Member]
Segment Information [Abstract]
Revenue			364,699	352,450	254,922
SPINRAZA Royalties [Member]
Segment Information [Abstract]
Revenue			286,583	292,992	237,930
Product Sales, Net [Member]
Segment Information [Abstract]
Revenue			69,999	42,253	2,237
Licensing and Other Royalty Revenue [Member]
Segment Information [Abstract]
Revenue			8,117	17,205	14,755
R&D Revenue Under Collaborative Agreements [Member]
Segment Information [Abstract]
Revenue			364,565	770,149	344,752
Operating Segments [Member] | Ionis Core [Member]
Segment Information [Abstract]
Revenue			622,941	858,646	641,944
Total operating expenses			563,647	523,207	380,212
Income (loss) from operations			59,294	335,439	261,732
Total assets	2,792,222	3,478,081	2,792,222	3,478,081
Operating Segments [Member] | Ionis Core [Member] | Commercial Revenue [Member]
Segment Information [Abstract]
Revenue			297,917	305,608	240,685
Operating Segments [Member] | Ionis Core [Member] | SPINRAZA Royalties [Member]
Segment Information [Abstract]
Revenue			286,583	292,992	237,930
Operating Segments [Member] | Ionis Core [Member] | Product Sales, Net [Member]
Segment Information [Abstract]
Revenue			0	0	0
Operating Segments [Member] | Ionis Core [Member] | Licensing and Other Royalty Revenue [Member]
Segment Information [Abstract]
Revenue			11,334	12,616	2,755
Operating Segments [Member] | Ionis Core [Member] | R&D Revenue Under Collaborative Agreements [Member]
Segment Information [Abstract]
Revenue			325,024	553,038	401,259
Operating Segments [Member] | Akcea Therapeutics [Member]
Segment Information [Abstract]
Revenue			152,320	488,543	64,867
Total operating expenses			389,575	450,469	295,683
Income (loss) from operations			(237,255)	38,074	(230,816)
Total assets	435,824	599,250	435,824	599,250
Operating Segments [Member] | Akcea Therapeutics [Member] | Commercial Revenue [Member]
Segment Information [Abstract]
Revenue			69,999	52,425	14,237
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			69,999	42,253	2,237
Operating Segments [Member] | Akcea Therapeutics [Member] | Licensing and Other Royalty Revenue [Member]
Segment Information [Abstract]
Revenue			0	10,172	12,000
Operating Segments [Member] | Akcea Therapeutics [Member] | R&D Revenue Under Collaborative Agreements [Member]
Segment Information [Abstract]
Revenue			82,321	436,118	50,630
Elimination of Intercompany Activity [Member]
Segment Information [Abstract]
Revenue			(45,997)	(224,590)	(107,137)
Total operating expenses			(51,876)	(216,960)	(14,849)
Income (loss) from operations			5,879	(7,630)	(92,288)
Total assets	$ (838,291)	$ (844,219)	(838,291)	(844,219)
Elimination of Intercompany Activity [Member] | Commercial Revenue [Member]
Segment Information [Abstract]
Revenue			(3,217)	(5,583)	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	0
Elimination of Intercompany Activity [Member] | Licensing and Other Royalty Revenue [Member]
Segment Information [Abstract]
Revenue			(3,217)	(5,583)	0
Elimination of Intercompany Activity [Member] | R&D Revenue Under Collaborative Agreements [Member]
Segment Information [Abstract]
Revenue			$ (42,780)	$ (219,007)	$ (107,137)