Significant Accounting Policies, Basic and Diluted Net Income (Loss) per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Basic Net Income (Loss) per Share [Abstract]
Basic net income (loss) per share (in dollars per share)	$ (0.22)	$ 0.19	$ (0.80)	$ 0.81
Net income (loss)	$ (30,944)	$ 26,163	$ (111,015)	$ 109,730
Net income (loss) available to Ionis common shareholders	$ (31,048)	$ 26,158	$ (111,225)	$ 112,764
Weighted average shares outstanding (in shares)	139,708	140,551	139,497	139,800
Income available to Ionis common shareholders		$ 26,158		$ 112,764
Shares issuable related to our Employee Stock Purchase Plan (in shares)		20		20
Shares used in computing diluted net income per share (in shares)	139,708	143,408	139,497	142,821
Diluted net income (loss) per share (in dollars per share)	$ (0.22)	$ 0.18	$ (0.80)	$ 0.79
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%
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)		1,993		2,208
Restricted Stock Awards [Member]
Basic Net Income (Loss) per Share [Abstract]
Shares issuable related to stock-based compensation (in shares)		844		793
Ionis [Member]
Basic Net Income (Loss) per Share [Abstract]
Net income (loss)	$ 6,774	$ 49,930	$ (3,049)	$ 140,938
Akcea [Member]
Basic Net Income (Loss) per Share [Abstract]
Net income (loss)	$ (37,822)	$ (23,772)	$ (108,176)	$ (28,174)
Akcea [Member] | Common Stock [Member]
Basic Net Income (Loss) per Share [Abstract]
Weighted average shares owned in Akcea (in shares)	77,095	70,221	77,095	69,681
Basic net income (loss) per share (in dollars per share)	$ (0.49)	$ (0.34)	$ (1.40)	$ (0.40)
Net income (loss)	$ (37,822)	$ (23,772)	$ (108,176)	$ (28,174)