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Segment Information (Details) $ in Thousands	3 Months Ended		9 Months Ended
Segment Information (Details) $ in Thousands	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) Segment	Sep. 30, 2018 USD ($)	Mar. 31, 2019	Dec. 31, 2018 USD ($)	Apr. 30, 2018	Jul. 31, 2017	Jun. 30, 2017
Segment Information [Abstract]
Number of reportable segments \| Segment			2
Segment Information [Abstract]
Revenue	$ 167,892	$ 145,395	$ 628,918	$ 407,559
Total operating expenses	165,369	163,967	523,689	479,716
Income (loss) from operations	2,523	(18,572)	105,229	(72,157)
Total assets as of current period	2,894,989		2,894,989			$ 2,667,784
Commercial Revenue [Member]
Segment Information [Abstract]
Revenue	95,699	82,756	252,085	181,975
SPINRAZA Royalties [Member]
Segment Information [Abstract]
Revenue	81,672	70,010	211,884	167,743
Product Sales, Net [Member]
Segment Information [Abstract]
Revenue	11,945	0	28,563	0
Licensing and Other Royalties [Member]
Segment Information [Abstract]
Revenue	2,082	12,746	11,638	14,232
R&D Revenue Under Collaborative Agreements [Member]
Segment Information [Abstract]
Revenue	$ 72,193	62,639	$ 376,833	225,584
Akcea [Member]
Segment Information [Abstract]
Percentage ownership	75.00%		75.00%		76.00%	75.00%	68.00%	68.00%	100.00%
Operating Segments [Member] \| Ionis Core [Member]
Segment Information [Abstract]
Revenue	$ 152,783	178,061	$ 514,862	410,025
Total operating expenses	119,147	87,664	355,437	279,084
Income (loss) from operations	33,636	90,397	159,425	130,941
Total assets as of current period	3,233,175		3,233,175			$ 2,975,491
Operating Segments [Member] \| Ionis Core [Member] \| Commercial Revenue [Member]
Segment Information [Abstract]
Revenue	83,618	77,956	220,350	177,175
Operating Segments [Member] \| Ionis Core [Member] \| SPINRAZA Royalties [Member]
Segment Information [Abstract]
Revenue	81,672	70,010	211,884	167,743
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 Royalties [Member]
Segment Information [Abstract]
Revenue	1,946	7,946	8,466	9,432
Operating Segments [Member] \| Ionis Core [Member] \| R&D Revenue Under Collaborative Agreements [Member]
Segment Information [Abstract]
Revenue	69,165	100,105	294,512	232,850
Operating Segments [Member] \| Akcea Therapeutics [Member]
Segment Information [Abstract]
Revenue	20,624	19,241	211,063	54,670
Total operating expenses	53,215	84,249	256,152	213,428
Income (loss) from operations	(32,591)	(65,008)	(45,089)	(158,758)
Total assets as of current period	383,167		383,167			365,261
Operating Segments [Member] \| Akcea Therapeutics [Member] \| Commercial Revenue [Member]
Segment Information [Abstract]
Revenue	12,081	12,000	34,735	12,000
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	11,945		28,563
Operating Segments [Member] \| Akcea Therapeutics [Member] \| Licensing and Other Royalties [Member]
Segment Information [Abstract]
Revenue	136	12,000	6,172	12,000
Operating Segments [Member] \| Akcea Therapeutics [Member] \| R&D Revenue Under Collaborative Agreements [Member]
Segment Information [Abstract]
Revenue	8,543	7,241	176,328	42,670
Elimination of Intercompany Activity [Member]
Segment Information [Abstract]
Revenue	(5,515)	(51,907)	(97,007)	(57,136)
Total operating expenses	(6,993)	(7,946)	(87,900)	(12,796)
Income (loss) from operations	1,478	(43,961)	(9,107)	(44,340)
Total assets as of current period	(721,353)		(721,353)			$ (672,968)
Elimination of Intercompany Activity [Member] \| Commercial Revenue [Member]
Segment Information [Abstract]
Revenue	0	(7,200)	(3,000)	(7,200)
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
Elimination of Intercompany Activity [Member] \| Licensing and Other Royalties [Member]
Segment Information [Abstract]
Revenue	0	(7,200)	(3,000)	(7,200)
Elimination of Intercompany Activity [Member] \| R&D Revenue Under Collaborative Agreements [Member]
Segment Information [Abstract]
Revenue	$ (5,515)	$ (44,707)	$ (94,007)	$ (49,936)