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Segment Information (Details) $ in Thousands	3 Months Ended
Segment Information (Details) $ in Thousands	Mar. 31, 2019 USD ($) Segment	Mar. 31, 2018 USD ($)	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	$ 297,214	$ 144,419
Total operating expenses	175,679	147,720
Income (loss) from operations	121,535	(3,301)
Total assets as of current period	2,835,889		$ 2,667,784
Commercial Revenue [Member]
Segment Information [Abstract]
Revenue	68,088	42,023
SPINRAZA Royalties [Member]
Segment Information [Abstract]
Revenue	59,711	41,081
TEGSEDI Product Sales, Net [Member]
Segment Information [Abstract]
Revenue	6,754	0
Licensing and Other Royalties [Member]
Segment Information [Abstract]
Revenue	1,623	942
R&D Revenue Under Collaborative Agreements [Member]
Segment Information [Abstract]
Revenue	$ 229,126	102,396
Akcea [Member]
Segment Information [Abstract]
Percentage ownership	76.00%		75.00%	68.00%	68.00%	100.00%
Operating Segments [Member] \| Ionis Core [Member]
Segment Information [Abstract]
Revenue	$ 221,890	132,540
Total operating expenses	114,515	105,544
Income (loss) from operations	107,375	26,996
Total assets as of current period	3,112,235		$ 2,975,491
Operating Segments [Member] \| Ionis Core [Member] \| Commercial Revenue [Member]
Segment Information [Abstract]
Revenue	61,334	42,023
Operating Segments [Member] \| Ionis Core [Member] \| SPINRAZA Royalties [Member]
Segment Information [Abstract]
Revenue	59,711	41,081
Operating Segments [Member] \| Ionis Core [Member] \| TEGSEDI Product Sales, Net [Member]
Segment Information [Abstract]
Revenue	0
Operating Segments [Member] \| Ionis Core [Member] \| Licensing and Other Royalties [Member]
Segment Information [Abstract]
Revenue	1,623	942
Operating Segments [Member] \| Ionis Core [Member] \| R&D Revenue Under Collaborative Agreements [Member]
Segment Information [Abstract]
Revenue	160,556	90,517
Operating Segments [Member] \| Akcea Therapeutics [Member]
Segment Information [Abstract]
Revenue	163,816	17,108
Total operating expenses	137,610	47,435
Income (loss) from operations	26,206	(30,327)
Total assets as of current period	458,717		365,261
Operating Segments [Member] \| Akcea Therapeutics [Member] \| Commercial Revenue [Member]
Segment Information [Abstract]
Revenue	6,754	0
Operating Segments [Member] \| Akcea Therapeutics [Member] \| SPINRAZA Royalties [Member]
Segment Information [Abstract]
Revenue	0	0
Operating Segments [Member] \| Akcea Therapeutics [Member] \| TEGSEDI Product Sales, Net [Member]
Segment Information [Abstract]
Revenue	6,754
Operating Segments [Member] \| Akcea Therapeutics [Member] \| Licensing and Other Royalties [Member]
Segment Information [Abstract]
Revenue	0	0
Operating Segments [Member] \| Akcea Therapeutics [Member] \| R&D Revenue Under Collaborative Agreements [Member]
Segment Information [Abstract]
Revenue	157,062	17,108
Elimination of Intercompany Activity [Member]
Segment Information [Abstract]
Revenue	(88,492)	(5,229)
Total operating expenses	(76,446)	(5,259)
Income (loss) from operations	(12,046)	30
Total assets as of current period	(735,063)		$ (672,968)
Elimination of Intercompany Activity [Member] \| Commercial Revenue [Member]
Segment Information [Abstract]
Revenue	0	0
Elimination of Intercompany Activity [Member] \| SPINRAZA Royalties [Member]
Segment Information [Abstract]
Revenue	0	0
Elimination of Intercompany Activity [Member] \| TEGSEDI Product Sales, Net [Member]
Segment Information [Abstract]
Revenue	0
Elimination of Intercompany Activity [Member] \| Licensing and Other Royalties [Member]
Segment Information [Abstract]
Revenue	0	0
Elimination of Intercompany Activity [Member] \| R&D Revenue Under Collaborative Agreements [Member]
Segment Information [Abstract]
Revenue	$ (88,492)	$ (5,229)