Significant Accounting Policies, Basic and Diluted Net Loss per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Basic Net Loss per Share [Abstract]
Basic net loss per share (in dollars per share)	$ (0.64)	$ (0.28)
Net loss	$ (89,869)	$ (39,628)
Net income (loss) available to Ionis common shareholders		$ (39,706)
Weighted average shares outstanding (in shares)	140,770	139,429
0.125 Percent Convertible Senior Notes [Member]
Basic Net Loss per Share [Abstract]
Interest rate on convertible senior notes	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%
Ionis [Member]
Basic Net Loss per Share [Abstract]
Net loss		$ (7,032)
Akcea [Member]
Basic Net Loss per Share [Abstract]
Percentage ownership	100.00%
Net loss		$ (32,674)
Akcea [Member] | Common Stock [Member]
Basic Net Loss per Share [Abstract]
Weighted average shares owned in Akcea (in shares)		77,095
Basic net loss per share (in dollars per share)		$ (0.42)
Net loss		$ (32,674)