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, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Basic Net Income (Loss) per Share [Abstract]
Basic net income (loss) per share (in dollars per share)	$ (0.23)	$ (0.01)	$ (0.58)	$ 0.62
Net income (loss)	$ (31,845)	$ (876)	$ (80,071)	$ 83,567
Net income (loss) available to Ionis common shareholders	$ (31,858)	$ (933)	$ (80,170)	$ 86,628
Weighted average shares outstanding (in shares)	139,352	140,247	139,391	139,419
Income available to Ionis common shareholders				$ 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)	139,352	140,247	139,391	142,499
Diluted net income (loss) per share (in dollars per share)	$ (0.23)	$ (0.01)	$ (0.58)	$ 0.61
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)				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)	$ 5,807	$ 27,311	$ (9,822)	$ 91,008
Akcea [Member]
Basic Net Income (Loss) per Share [Abstract]
Net income (loss)	$ (37,665)	$ (28,244)	$ (70,348)	$ (4,380)
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	69,406
Basic net income (loss) per share (in dollars per share)	$ (0.49)	$ (0.40)	$ (0.91)	$ (0.06)
Net income (loss)	$ (37,665)	$ (28,244)	$ (70,348)	$ (4,380)