Fourth Quarter Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	1 Months Ended	2 Months Ended	3 Months Ended				12 Months Ended
	Oct. 31, 2020	Dec. 31, 2020	Dec. 31, 2021		Dec. 31, 2020		Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Nov. 30, 2021	Apr. 30, 2021	Dec. 31, 2016	Nov. 30, 2014
Fourth Quarter Financial Data (Unaudited) [Abstract]
Revenue			$ 440,006		$ 290,281
Operating expenses			219,403		312,945		$ 840,642	$ 901,346	$ 756,716
Income (loss) from operations			220,603		(22,664)		(30,186)	(172,082)	365,883
Net income (loss)			224,613		(355,687)		(28,597)	(479,743)	287,259
Net income (loss) attributable to Ionis Pharmaceuticals, Inc. common stockholders			$ 224,613		$ (354,532)		$ (28,597)	$ (444,263)	$ 278,143
Basic net income (loss) per share (in dollars per share)			$ 1.59	[1],[2]	$ (2.54)	[1],[2]	$ (0.20)	$ (3.18)	$ 2.00
Diluted net income (loss) per share (in dollars per share)			$ 1.41	[1],[3]	$ (2.54)	[1]	$ (0.20)	$ (3.18)	$ 1.90
Income (Numerator) [Abstract]
Income (loss) available to Ionis common stockholders			$ 224,612		$ (356,008)			$ (444,464)	$ 280,560
Net income available to Ionis common shareholders			$ 226,210						$ 290,947
Shares (Denominator) [Abstract]
Shares used in computing basic net income (loss) per share (in shares)			141,205		139,956		141,021	139,612	139,998
Effect of Diluted Securities [Abstract]
Shares issuable related to our ESPP (in shares)			34						18
Shares used in computing diluted net income per share (in shares)			160,340				141,021	139,612	153,164
0 Percent Convertible Senior Notes [Member]
Fourth Quarter Financial Data (Unaudited) [Abstract]
Interest rate on convertible senior notes			0.00%				0.00%		0.00%		0.00%
Income (Numerator) [Abstract]
Shares issuable related to our convertible notes			$ 777
Effect of Diluted Securities [Abstract]
Shares issuable related to our convertible notes (in shares)			10,936
0.125 Percent Convertible Senior Notes [Member]
Fourth Quarter Financial Data (Unaudited) [Abstract]
Interest rate on convertible senior notes		0.125%	0.125%		0.125%		0.125%	0.125%	0.125%
Income (Numerator) [Abstract]
Shares issuable related to our convertible notes			$ 716						$ 860
Effect of Diluted Securities [Abstract]
Shares issuable related to our convertible notes (in shares)			6,590						217
1 Percent Convertible Senior Notes [Member]
Fourth Quarter Financial Data (Unaudited) [Abstract]
Interest rate on convertible senior notes		1.00%	1.00%		1.00%		1.00%	1.00%	1.00%	1.00%	1.00%	1.00%	1.00%
Income (Numerator) [Abstract]
Shares issuable related to our convertible notes			$ 105						$ 9,527
Effect of Diluted Securities [Abstract]
Shares issuable related to our convertible notes (in shares)			464						10,075
Stock Options [Member]
Effect of Diluted Securities [Abstract]
Shares issuable related to stock-based compensation (in shares)			46						2,090
Restricted Stock Awards [Member]
Effect of Diluted Securities [Abstract]
Shares issuable related to stock-based compensation (in shares)			1,065						766
Ionis [Member]
Fourth Quarter Financial Data (Unaudited) [Abstract]
Net income (loss) attributable to Ionis Pharmaceuticals, Inc. common stockholders					$ (266,418)			$ (246,702)	$ 246,487
Akcea [Member]
Fourth Quarter Financial Data (Unaudited) [Abstract]
Net income (loss) attributable to Ionis Pharmaceuticals, Inc. common stockholders	$ (3,603)	$ (85,987)			$ (89,590)
Basic net income (loss) per share (in dollars per share)					$ (0.05)
Weighted average shares owned in Akcea (in shares)					77,095

[1] We compute net income (loss) per share independently for each quarter during the year.

[2] As discussed in Note 1, Organization and Significant Accounting Policies, we compute basic net income (loss) per share by dividing the total net income (loss) attributable to our common stockholders by our weighted-average number of common shares outstanding during the period. Our basic net income per share for the fourth quarter of 2021 was $1.59.Our basic net loss per share calculation for the fourth quarter of 2020 considered our net loss for Ionis on a stand-alone basis plus our share of Akcea’s net loss for the period. To calculate the portion of Akcea’s net loss attributable to our ownership, we multiplied Akcea’s loss per share by the weighted average shares we owned in Akcea during the period. As a result of this calculation, our total net loss available to Ionis common stockholders for the calculation of net loss per share is different than net loss attributable to Ionis Pharmaceuticals, Inc. common stockholders in the consolidated statements of operations.

[3] We had net income available to Ionis common stockholders for the fourth quarter of 2021. As a result, we computed diluted net income per share using the weighted-average number of common shares and dilutive common equivalent shares outstanding during the period