Segment Information (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) Segment	Jun. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)	Jun. 30, 2017
Segment Information [Abstract]
Number of reportable segments | Segment			2
Segment Information [Abstract]
Revenue	$ 145,537	$ 163,813	$ 278,905	$ 461,026
Total operating expenses	197,291	182,640	391,784	358,320
Income (loss) from operations	(51,754)	(18,827)	(112,879)	102,706
Total assets	3,078,950		3,078,950		$ 3,233,112
Commercial Revenue [Member]
Segment Information [Abstract]
Revenue	89,734	88,299	173,696	156,386
SPINRAZA Royalties [Member]
Segment Information [Abstract]
Revenue	71,746	70,502	137,754	130,212
Product Sales, Net [Member]
Segment Information [Abstract]
Revenue	16,364	9,865	31,523	16,619
Licensing and Other Royalty Revenue [Member]
Segment Information [Abstract]
Revenue	1,624	7,932	4,419	9,555
R&D Revenue Under Collaborative Agreements [Member]
Segment Information [Abstract]
Revenue	$ 55,803	75,514	$ 105,209	304,640
Akcea [Member]
Segment Information [Abstract]
Percentage ownership	76.00%		76.00%			100.00%
Operating Segments [Member] | Ionis Core [Member]
Segment Information [Abstract]
Revenue	$ 129,053	140,189	$ 247,150	362,078
Total operating expenses	131,177	121,774	266,602	236,290
Income (loss) from operations	(2,124)	18,415	(19,452)	125,788
Total assets	3,397,604		3,397,604		3,478,081
Operating Segments [Member] | Ionis Core [Member] | Commercial Revenue [Member]
Segment Information [Abstract]
Revenue	73,983	75,398	143,589	136,731
Operating Segments [Member] | Ionis Core [Member] | SPINRAZA Royalties [Member]
Segment Information [Abstract]
Revenue	71,746	70,502	137,754	130,212
Operating Segments [Member] | Ionis Core [Member] | Product Sales, Net [Member]
Segment Information [Abstract]
Revenue	0	0	0	0
Operating Segments [Member] | Ionis Core [Member] | Licensing and Other Royalty Revenue [Member]
Segment Information [Abstract]
Revenue	2,237	4,896	5,835	6,519
Operating Segments [Member] | Ionis Core [Member] | R&D Revenue Under Collaborative Agreements [Member]
Segment Information [Abstract]
Revenue	55,070	64,791	103,561	225,347
Operating Segments [Member] | Akcea Therapeutics [Member]
Segment Information [Abstract]
Revenue	22,377	26,624	38,451	190,440
Total operating expenses	73,468	65,328	134,800	202,938
Income (loss) from operations	(51,091)	(38,704)	(96,349)	(12,498)
Total assets	527,937		527,937		599,250
Operating Segments [Member] | Akcea Therapeutics [Member] | Commercial Revenue [Member]
Segment Information [Abstract]
Revenue	16,364	15,901	31,523	22,655
Operating Segments [Member] | Akcea Therapeutics [Member] | SPINRAZA Royalties [Member]
Segment Information [Abstract]
Revenue	0	0	0	0
Operating Segments [Member] | Akcea Therapeutics [Member] | Product Sales, Net [Member]
Segment Information [Abstract]
Revenue	16,364	9,865	31,523	16,619
Operating Segments [Member] | Akcea Therapeutics [Member] | Licensing and Other Royalty Revenue [Member]
Segment Information [Abstract]
Revenue	0	6,036	0	6,036
Operating Segments [Member] | Akcea Therapeutics [Member] | R&D Revenue Under Collaborative Agreements [Member]
Segment Information [Abstract]
Revenue	6,013	10,723	6,928	167,785
Elimination of Intercompany Activity [Member]
Segment Information [Abstract]
Revenue	(5,893)	(3,000)	(6,696)	(91,492)
Total operating expenses	(7,354)	(4,462)	(9,618)	(80,908)
Income (loss) from operations	1,461	1,462	2,922	(10,584)
Total assets	(846,591)		(846,591)		$ (844,219)
Elimination of Intercompany Activity [Member] | Commercial Revenue [Member]
Segment Information [Abstract]
Revenue	(613)	(3,000)	(1,416)	(3,000)
Elimination of Intercompany Activity [Member] | SPINRAZA Royalties [Member]
Segment Information [Abstract]
Revenue	0	0	0	0
Elimination of Intercompany Activity [Member] | Product Sales, Net [Member]
Segment Information [Abstract]
Revenue	0	0	0	0
Elimination of Intercompany Activity [Member] | Licensing and Other Royalty Revenue [Member]
Segment Information [Abstract]
Revenue	(613)	(3,000)	(1,416)	(3,000)
Elimination of Intercompany Activity [Member] | R&D Revenue Under Collaborative Agreements [Member]
Segment Information [Abstract]
Revenue	$ (5,280)	$ 0	$ (5,280)	$ (88,492)