Segment Information (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) Segment	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)	Jun. 30, 2017
Segment Information [Abstract]
Number of reportable segments | Segment			2
Segment Information [Abstract]
Revenue	$ 160,079	$ 167,892	$ 438,983	$ 628,918
Total operating expenses	196,616	165,369	588,399	523,689
Income (loss) from operations	(36,537)	2,523	(149,416)	105,229
Total assets	3,095,701		3,095,701		$ 3,233,112
Commercial Revenue [Member]
Segment Information [Abstract]
Revenue	95,340	95,699	269,035	252,085
SPINRAZA Royalties [Member]
Segment Information [Abstract]
Revenue	74,171	81,672	211,925	211,884
Product Sales, Net [Member]
Segment Information [Abstract]
Revenue	19,040	11,945	50,562	28,563
Licensing and Other Royalty Revenue [Member]
Segment Information [Abstract]
Revenue	2,129	2,082	6,548	11,638
R&D Revenue Under Collaborative Agreements [Member]
Segment Information [Abstract]
Revenue	$ 64,739	72,193	$ 169,948	376,833
Akcea [Member]
Segment Information [Abstract]
Percentage ownership	76.00%		76.00%			100.00%
Operating Segments [Member] | Ionis Core [Member]
Segment Information [Abstract]
Revenue	$ 141,455	152,783	$ 388,604	514,862
Total operating expenses	129,657	119,147	396,259	355,437
Income (loss) from operations	11,798	33,636	(7,655)	159,425
Total assets	3,436,990		3,436,990		3,478,081
Operating Segments [Member] | Ionis Core [Member] | Commercial Revenue [Member]
Segment Information [Abstract]
Revenue	77,146	83,618	220,734	220,350
Operating Segments [Member] | Ionis Core [Member] | SPINRAZA Royalties [Member]
Segment Information [Abstract]
Revenue	74,171	81,672	211,925	211,884
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	2,975	1,946	8,809	8,466
Operating Segments [Member] | Ionis Core [Member] | R&D Revenue Under Collaborative Agreements [Member]
Segment Information [Abstract]
Revenue	64,309	69,165	167,870	294,512
Operating Segments [Member] | Akcea Therapeutics [Member]
Segment Information [Abstract]
Revenue	19,470	20,624	57,920	211,063
Total operating expenses	69,283	53,215	204,084	256,152
Income (loss) from operations	(49,813)	(32,591)	(146,164)	(45,089)
Total assets	507,977		507,977		599,250
Operating Segments [Member] | Akcea Therapeutics [Member] | Commercial Revenue [Member]
Segment Information [Abstract]
Revenue	19,040	12,081	50,562	34,735
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	19,040	11,945	50,562	28,563
Operating Segments [Member] | Akcea Therapeutics [Member] | Licensing and Other Royalty Revenue [Member]
Segment Information [Abstract]
Revenue	0	136	0	6,172
Operating Segments [Member] | Akcea Therapeutics [Member] | R&D Revenue Under Collaborative Agreements [Member]
Segment Information [Abstract]
Revenue	430	8,543	7,358	176,328
Elimination of Intercompany Activity [Member]
Segment Information [Abstract]
Revenue	(846)	(5,515)	(7,541)	(97,007)
Total operating expenses	(2,324)	(6,993)	(11,944)	(87,900)
Income (loss) from operations	1,478	1,478	4,403	(9,107)
Total assets	(849,266)		(849,266)		$ (844,219)
Elimination of Intercompany Activity [Member] | Commercial Revenue [Member]
Segment Information [Abstract]
Revenue	(846)	0	(2,261)	(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	(846)	0	(2,261)	(3,000)
Elimination of Intercompany Activity [Member] | R&D Revenue Under Collaborative Agreements [Member]
Segment Information [Abstract]
Revenue	$ 0	$ (5,515)	$ (5,280)	$ (94,007)