Significant Accounting Policies, Basic and Diluted Net Loss per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Mar. 31, 2021	Dec. 31, 2020
Basic Net Loss per Share [Abstract]
Basic net loss per share (in dollars per share)	$ (0.57)	$ (0.18)	$ (1.21)	$ (0.47)
Net income (loss)	$ (80,875)	$ (25,636)	$ (170,744)	$ (65,264)
Net loss available to Ionis common shareholders		$ (25,649)		$ (65,363)
Weighted average shares outstanding (in shares)	140,962	139,352	140,866	139,391
0.125 Percent Convertible Senior Notes [Member]
Basic Net Loss per Share [Abstract]
Interest rate on convertible senior notes	0.125%	0.125%	0.125%	0.125%		0.125%
1 Percent Convertible Senior Notes [Member]
Basic Net Loss per Share [Abstract]
Interest rate on convertible senior notes	1.00%	1.00%	1.00%	1.00%		1.00%
0 Percent Convertible Senior Notes [Member]
Basic Net Loss per Share [Abstract]
Interest rate on convertible senior notes	0.00%	0.00%	0.00%	0.00%
Ionis [Member]
Basic Net Loss per Share [Abstract]
Net income (loss)		$ 12,016		$ 4,985
Akcea [Member]
Basic Net Loss per Share [Abstract]
Percentage ownership					100.00%
Net income (loss)		$ (37,665)		$ (70,348)
Akcea [Member] | Common Stock [Member]
Basic Net Loss per Share [Abstract]
Weighted average shares owned in Akcea (in shares)		77,095		77,095
Basic net loss per share (in dollars per share)		$ (0.49)		$ (0.91)
Net income (loss)		$ (37,665)		$ (70,348)