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Segment Information (Details) $ in Thousands	3 Months Ended
Segment Information (Details) $ in Thousands	Mar. 31, 2020 USD ($) Segment	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($)	Jun. 30, 2017
Segment Information [Abstract]
Number of reportable segments \| Segment	2
Segment Information [Abstract]
Revenue	$ 133,367	$ 297,214
Total operating expenses	194,494	175,679
Income (loss) from operations	(61,127)	121,535
Total assets as of current period	3,090,913		$ 3,233,112
Commercial Revenue [Member]
Segment Information [Abstract]
Revenue	83,961	68,088
SPINRAZA Royalties [Member]
Segment Information [Abstract]
Revenue	66,008	59,711
Product Sales, Net [Member]
Segment Information [Abstract]
Revenue	15,159	6,754
Licensing and Other Royalty Revenue [Member]
Segment Information [Abstract]
Revenue	2,794	1,623
R&D Revenue Under Collaborative Agreements [Member]
Segment Information [Abstract]
Revenue	$ 49,406	229,126
Akcea [Member]
Segment Information [Abstract]
Percentage ownership	76.00%			100.00%
Operating Segments [Member] \| Ionis Core [Member]
Segment Information [Abstract]
Revenue	$ 118,097	221,890
Total operating expenses	135,426	114,515
Income (loss) from operations	(17,329)	107,375
Total assets as of current period	3,376,169		3,478,081
Operating Segments [Member] \| Ionis Core [Member] \| Commercial Revenue [Member]
Segment Information [Abstract]
Revenue	69,606	61,334
Operating Segments [Member] \| Ionis Core [Member] \| SPINRAZA Royalties [Member]
Segment Information [Abstract]
Revenue	66,008	59,711
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	3,598	1,623
Operating Segments [Member] \| Ionis Core [Member] \| R&D Revenue Under Collaborative Agreements [Member]
Segment Information [Abstract]
Revenue	48,491	160,556
Operating Segments [Member] \| Akcea Therapeutics [Member]
Segment Information [Abstract]
Revenue	16,074	163,816
Total operating expenses	61,334	137,610
Income (loss) from operations	(45,260)	26,206
Total assets as of current period	559,921		599,250
Operating Segments [Member] \| Akcea Therapeutics [Member] \| Commercial Revenue [Member]
Segment Information [Abstract]
Revenue	15,159	6,754
Operating Segments [Member] \| Akcea Therapeutics [Member] \| SPINRAZA Royalties [Member]
Segment Information [Abstract]
Revenue	0	0
Operating Segments [Member] \| Akcea Therapeutics [Member] \| Product Sales, Net [Member]
Segment Information [Abstract]
Revenue	15,159	6,754
Operating Segments [Member] \| Akcea Therapeutics [Member] \| Licensing and Other Royalty Revenue [Member]
Segment Information [Abstract]
Revenue	0	0
Operating Segments [Member] \| Akcea Therapeutics [Member] \| R&D Revenue Under Collaborative Agreements [Member]
Segment Information [Abstract]
Revenue	915	157,062
Elimination of Intercompany Activity [Member]
Segment Information [Abstract]
Revenue	(804)	(88,492)
Total operating expenses	(2,266)	(76,446)
Income (loss) from operations	1,462	(12,046)
Total assets as of current period	(845,177)		$ (844,219)
Elimination of Intercompany Activity [Member] \| Commercial Revenue [Member]
Segment Information [Abstract]
Revenue	(804)	0
Elimination of Intercompany Activity [Member] \| SPINRAZA Royalties [Member]
Segment Information [Abstract]
Revenue	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	(804)	0
Elimination of Intercompany Activity [Member] \| R&D Revenue Under Collaborative Agreements [Member]
Segment Information [Abstract]
Revenue	$ 0	$ (88,492)