Significant Accounting Policies, Basic and Diluted Net Loss per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Basic Net Loss per Share [Abstract]
Basic net loss per share (in dollars per share)	$ (0.58)	$ (0.18)	$ (1.80)	$ (0.64)
Net income (loss)	$ (82,466)	$ (24,467)	$ (253,210)	$ (89,731)
Net loss available to Ionis common shareholders		$ (24,571)		$ (89,941)
Weighted average shares outstanding (in shares)	141,139	139,708	140,958	139,497
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%
Ionis [Member]
Basic Net Loss per Share [Abstract]
Net income (loss)		$ 13,251		$ 18,235
Akcea [Member]
Basic Net Loss per Share [Abstract]
Percentage ownership	100.00%		100.00%
Net income (loss)		$ (37,822)		$ (108,176)
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)		$ (1.40)
Net income (loss)		$ (37,822)		$ (108,176)