Organization and Significant Accounting Policies, Basic and Diluted Net Income (Loss) per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	2 Months Ended	3 Months Ended				10 Months Ended	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2020		Dec. 31, 2019		Oct. 31, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Basic Net Income (Loss) per Share [Abstract]
Basic net income (loss) per share (in dollars per share)		$ (2.44)	[1],[2]	$ 1.31	[1],[2]		$ (3.23)	$ 2.12	$ 2.09
Net income (loss)		$ (340,271)		$ 184,415			$ (451,286)	$ 294,146	$ 273,741
Net income (loss) available to Ionis common stockholders		$ (340,272)		$ 183,795			$ (451,487)	$ 296,563	$ 276,868
Weighted average shares outstanding (in shares)		139,956		140,583			139,612	139,998	132,320
Net income available to Ionis common shareholders				$ 196,485				$ 296,563	$ 276,868
Shares issuable related to our ESPP (in shares)				18				18	6
Shares used in computing diluted net income per share (in shares)				153,303			139,612	142,872	134,056
Diluted net income per share (in dollars per share)		$ (2.44)	[2],[3]	$ 1.28	[2],[3]		$ (3.23)	$ 2.08	$ 2.07
0.125 Percent Convertible Senior Notes [Member]
Basic Net Income (Loss) per Share [Abstract]
Interest rate on convertible senior notes	0.125%	0.125%		0.125%			0.125%	0.125%
1 Percent Convertible Senior Notes [Member]
Basic Net Income (Loss) per Share [Abstract]
Interest rate on convertible senior notes	1.00%	1.00%		1.00%			1.00%	1.00%
Stock Options [Member]
Basic Net Income (Loss) per Share [Abstract]
Shares issuable related to stock-based compensation (in shares)				1,467				2,090	1,216
Restricted Stock Awards [Member]
Basic Net Income (Loss) per Share [Abstract]
Shares issuable related to stock-based compensation (in shares)				848				766	514
Ionis [Member]
Basic Net Income (Loss) per Share [Abstract]
Net income (loss)		$ (250,682)		$ 121,552			$ (253,725)	$ 262,490	$ 440,806
Akcea [Member]
Basic Net Income (Loss) per Share [Abstract]
Net income (loss)	$ (85,987)						$ (197,762)	$ 34,073	$ (163,938)
Akcea [Member] | Common Stock [Member]
Basic Net Income (Loss) per Share [Abstract]
Weighted average shares owned in Akcea (in shares)						77,095		70,100	59,812
Basic net income (loss) per share (in dollars per share)						$ (1.45)		$ 0.49	$ (2.74)
Net income (loss)						$ (111,775)		$ 34,073	$ (163,938)

[1] 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 (loss) per share calculation for each of the fourth quarters in 2020 and 2019 considered our net income (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 income (loss) available to Ionis common stockholders for the calculation of net income (loss) per share is different than net income (loss) attributable to Ionis Pharmaceuticals, Inc. common stockholders in the consolidated statements of operations.

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

[3] We had net income available to Ionis common stockholders for the fourth quarter of 2019. 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