Significant Accounting Policies, Basic and Diluted Net Income (Loss) per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Basic Net Income (Loss) per Share [Abstract]
Basic net income (loss) per share (in dollars per share)	$ (0.01)	$ (0.29)	$ 0.62	$ (0.30)
Net income (loss)	$ (876)	$ (40,355)	$ 83,567	$ (41,775)
Net income (loss) available to Ionis common shareholders	$ (933)	$ (37,932)	$ 86,628	$ (38,530)
Weighted average shares outstanding (in shares)	140,247	128,712	139,419	127,030
Income available to Ionis common shareholders, plus assumed conversions			$ 86,628
Shares issuable related to our Employee Stock Purchase Plan (in shares)			8
Shares used in computing diluted net income per share (in shares)	140,247	128,712	142,499	127,030
Diluted net income per share (in dollars per share)	$ (0.01)	$ (0.29)	$ 0.61	$ (0.30)
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)			2,327
Restricted Stock Awards [Member]
Basic Net Income (Loss) per Share [Abstract]
Shares issuable related to stock-based compensation (in shares)			745
Ionis [Member]
Basic Net Income (Loss) per Share [Abstract]
Net income (loss)	$ 27,311	$ 5,882	$ 91,008	$ 24,668
Akcea [Member]
Basic Net Income (Loss) per Share [Abstract]
Net income (loss)	$ (28,244)	$ (43,814)	$ (4,380)	$ (63,198)
Akcea [Member] | Common Stock [Member]
Basic Net Income (Loss) per Share [Abstract]
Weighted average shares owned in Akcea (in shares)	70,221	60,832	69,406	53,183
Basic net income (loss) per share (in dollars per share)	$ (0.40)	$ (0.72)	$ (0.06)	$ (1.19)
Net income (loss)	$ (28,244)	$ (43,814)	$ (4,380)	$ (63,198)