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, 2019	Mar. 31, 2018	Dec. 31, 2018
Basic Net Income (Loss) per Share [Abstract]
Basic net income (loss) per share (in dollars per share)	$ 0.63	$ (0.01)
Net income (loss)	$ 84,443	$ (1,420)
Net income (loss) available to Ionis common shareholders	$ 87,543	$ (1,212)
Weighted average shares outstanding (in shares)	138,582	125,330
Income available to Ionis common shareholders, plus assumed conversions	$ 87,543
Shares issuable related to our ESPP (in shares)	38
Shares used in computing diluted net income per share (in shares)	141,537	125,330
Diluted net income per share (in dollars per share)	$ 0.62	$ (0.01)
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)	$ 63,697	$ 18,785
Akcea [Member]
Basic Net Income (Loss) per Share [Abstract]
Net income (loss)	$ 23,846	$ (19,997)
Akcea [Member] | Common Stock [Member]
Basic Net Income (Loss) per Share [Abstract]
Weighted average shares owned in Akcea (in shares)	68,582	45,448
Basic net income (loss) per share (in dollars per share)	$ 0.35	$ (0.44)
Net income (loss)	$ 23,846	$ (19,997)