Significant Accounting Policies, Basic and Diluted Net Income (Loss) per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Basic Net Income (Loss) per Share [Abstract]
Basic net income per share (in dollars per share)	$ (0.35)	$ 0.63
Net income (loss)	$ (48,226)	$ 84,443
Net income (loss) available to Ionis common shareholders	$ (48,304)	$ 87,543
Weighted average shares outstanding (in shares)	139,429	138,582
Income available to Ionis common shareholders		$ 87,543
Shares issuable related to our Employee Stock Purchase Plan (in shares)		38
Shares used in computing diluted net income per share (in shares)	139,429	141,537
Diluted net income per share (in dollars per share)	$ (0.35)	$ 0.62
0.125 Percent Convertible Senior Notes [Member]
Basic Net Income (Loss) per Share [Abstract]
Interest rate on convertible senior notes	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%
Stock Options [Member]
Basic Net Income (Loss) per Share [Abstract]
Shares issuable related to stock-based compensation (in shares)		2,252
Restricted Stock Awards [Member]
Basic Net Income (Loss) per Share [Abstract]
Shares issuable related to stock-based compensation (in shares)		665
Ionis [Member]
Basic Net Income (Loss) per Share [Abstract]
Net income (loss)	$ (15,630)	$ 63,697
Akcea [Member]
Basic Net Income (Loss) per Share [Abstract]
Net income (loss)	$ (32,674)	$ 23,846
Akcea [Member] | Common Stock [Member]
Basic Net Income (Loss) per Share [Abstract]
Weighted average shares owned in Akcea (in shares)	77,095	68,582
Basic net income per share (in dollars per share)	$ (0.42)	$ 0.35
Net income (loss)	$ (32,674)	$ 23,846