CONDENSED CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) shares in Thousands, $ in Thousands	Common Stock [Member]	Additional Paid In Capital [Member]	Accumulated Other Comprehensive Loss [Member]	Accumulated Deficit [Member]	Total Ionis Stockholders' Equity [Member]	Noncontrolling Interest in Akcea Therapeutics, Inc. [Member]	Total
Balance at Dec. 31, 2019	$ 140	$ 1,985,650	$ (25,290)	$ (596,495)	$ 1,364,005	$ 213,453	$ 1,577,458
Balance (in shares) at Dec. 31, 2019	140,340
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	$ 0	0	0	(39,628)	(39,628)	0	(39,628)
Change in unrealized losses, net of tax	0	0	(1,954)	0	(1,954)	0	(1,954)
Foreign currency translation	0	0	9	0	9	0	9
Issuance of common stock in connection with employee stock plans	$ 0	7,652	0	0	7,652	0	7,652
Issuance of common stock in connection with employee stock plans (in shares)	606
Repurchases and retirements of common stock	$ (1)	0	0	(90,549)	(90,550)	0	(90,550)
Repurchases and retirements of common stock (in shares)	(1,478)
Stock-based compensation expense	$ 0	40,790	0	0	40,790	0	40,790
Payments of tax withholdings related to vesting of employee stock awards and exercise of employee stock options	$ 0	(11,603)	0	0	(11,603)	0	(11,603)
Payments of tax withholdings related to vesting of employee stock awards and exercise of employee stock options (in shares)	(186)
Noncontrolling interest in Akcea Therapeutics, Inc.	$ 0	(6,973)	0	0	(6,973)	(3,281)	(10,254)
Balance at Mar. 31, 2020	$ 139	2,015,516	(27,235)	(726,672)	1,261,748	210,172	1,471,920
Balance (in shares) at Mar. 31, 2020	139,282
Balance at Dec. 31, 2020	$ 140	1,895,519	(21,071)	(1,131,306)	743,282	0	743,282
Balance (in shares) at Dec. 31, 2020	140,366
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	$ 0	0	0	(89,869)	(89,869)	0	(89,869)
Change in unrealized losses, net of tax	0	0	(3,006)	0	(3,006)	0	(3,006)
Foreign currency translation	0	0	(126)	0	(126)	0	(126)
Issuance of common stock in connection with employee stock plans	$ 1	7,758	0	0	7,759	0	7,759
Issuance of common stock in connection with employee stock plans (in shares)	809
Stock-based compensation expense	$ 0	37,861	0	0	37,861	0	37,861
Payments of tax withholdings related to vesting of employee stock awards and exercise of employee stock options	$ 0	(15,337)	0	0	(15,337)	0	(15,337)
Payments of tax withholdings related to vesting of employee stock awards and exercise of employee stock options (in shares)	(251)
Balance at Mar. 31, 2021	$ 141	$ 1,925,801	$ (24,203)	$ (1,221,175)	$ 680,564	$ 0	$ 680,564
Balance (in shares) at Mar. 31, 2021	140,924