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, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Basic Net Income (Loss) per Share [Abstract]
Basic net income (loss) per share (in dollars per share)	$ 0.19	$ (0.03)	$ 0.81	$ (0.33)
Net income (loss)	$ 26,163	$ (4,559)	$ 109,730	$ (46,335)
Net income (loss) available to Ionis common shareholders	$ 26,158	$ (4,617)	$ 112,764	$ (43,163)
Weighted average shares outstanding (in shares)	140,551	137,346	139,800	130,507
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)	143,408	137,346	142,821	130,507
Diluted net income per share (in dollars per share)	$ 0.18	$ (0.03)	$ 0.79	$ (0.33)
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)	$ 49,930	$ 43,226	$ 140,938	$ 67,517
Akcea [Member]
Basic Net Income (Loss) per Share [Abstract]
Net income (loss)	$ (23,772)	$ (47,843)	$ (28,174)	$ (110,680)
Akcea [Member] | Common Stock [Member]
Basic Net Income (Loss) per Share [Abstract]
Weighted average shares owned in Akcea (in shares)	70,221	65,538	69,681	57,347
Basic net income (loss) per share (in dollars per share)	$ (0.34)	$ (0.73)	$ (0.40)	$ (1.93)
Net income (loss)	$ (23,772)	$ (47,843)	$ (28,174)	$ (110,680)