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	(89,731)	(89,731)	0	(89,731)
Change in unrealized gains (losses), net of tax	0	0	5,803	0	5,803	0	5,803
Foreign currency translation	0	0	357	0	357	0	357
Issuance of common stock in connection with employee stock plans	$ 1	29,449	0	0	29,450	0	29,450
Issuance of common stock in connection with employee stock plans (in shares)	1,141
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	135,077	0	0	135,077	0	135,077
Payments of tax withholdings related to vesting of employee stock awards and exercise of employee stock options	$ 0	(12,960)	0	0	(12,960)	0	(12,960)
Payments of tax withholdings related to vesting of employee stock awards and exercise of employee stock options (in shares)	(209)
Noncontrolling interest in Akcea Therapeutics, Inc.	$ 0	(43,665)	(442)	0	(44,107)	9,782	(34,325)
Balance at Sep. 30, 2020	$ 140	2,093,551	(19,572)	(776,775)	1,297,344	223,235	1,520,579
Balance (in shares) at Sep. 30, 2020	139,794
Balance at Jun. 30, 2020	$ 139	2,053,502	(16,440)	(752,308)	1,284,893	217,620	1,502,513
Balance (in shares) at Jun. 30, 2020	139,489
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	$ 0	0	0	(24,467)	(24,467)	0	(24,467)
Change in unrealized gains (losses), net of tax	0	0	(3,448)	0	(3,448)	0	(3,448)
Foreign currency translation	0	0	275	0	275	0	275
Issuance of common stock in connection with employee stock plans	$ 1	12,997	0	0	12,998	0	12,998
Issuance of common stock in connection with employee stock plans (in shares)	321
Stock-based compensation expense	$ 0	45,845	0	0	45,845	0	45,845
Payments of tax withholdings related to vesting of employee stock awards and exercise of employee stock options	$ 0	(990)	0	0	(990)	0	(990)
Payments of tax withholdings related to vesting of employee stock awards and exercise of employee stock options (in shares)	(16)
Noncontrolling interest in Akcea Therapeutics, Inc.	$ 0	(17,803)	41	0	(17,762)	5,615	(12,147)
Balance at Sep. 30, 2020	$ 140	2,093,551	(19,572)	(776,775)	1,297,344	223,235	1,520,579
Balance (in shares) at Sep. 30, 2020	139,794
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	(253,210)	(253,210)	0	(253,210)
Change in unrealized gains (losses), net of tax	0	0	(6,321)	0	(6,321)	0	(6,321)
Foreign currency translation	0	0	(45)	0	(45)	0	(45)
Issuance of common stock in connection with employee stock plans	$ 1	11,563	0	0	11,564	0	11,564
Issuance of common stock in connection with employee stock plans (in shares)	1,094
Issuance of warrants	$ 0	89,752	0	0	89,752	0	89,752
Purchase of note hedges	0	(136,620)	0	0	(136,620)	0	(136,620)
Stock-based compensation expense	0	98,419	0	0	98,419	0	98,419
Payments of tax withholdings related to vesting of employee stock awards and exercise of employee stock options	$ 0	(16,285)	0	0	(16,285)	0	(16,285)
Payments of tax withholdings related to vesting of employee stock awards and exercise of employee stock options (in shares)	(276)
Balance at Sep. 30, 2021	$ 141	1,942,348	(27,437)	(1,384,516)	530,536	0	530,536
Balance (in shares) at Sep. 30, 2021	141,184
Balance at Jun. 30, 2021	$ 141	1,910,379	(25,796)	(1,302,050)	582,674	0	582,674
Balance (in shares) at Jun. 30, 2021	141,022
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	$ 0	0	0	(82,466)	(82,466)	0	(82,466)
Change in unrealized gains (losses), net of tax	0	0	(1,618)	0	(1,618)	0	(1,618)
Foreign currency translation	0	0	(23)	0	(23)	0	(23)
Issuance of common stock in connection with employee stock plans	$ 0	1,922	0	0	1,922	0	1,922
Issuance of common stock in connection with employee stock plans (in shares)	176
Stock-based compensation expense	$ 0	30,537	0	0	30,537	0	30,537
Payments of tax withholdings related to vesting of employee stock awards and exercise of employee stock options	$ 0	(490)	0	0	(490)	0	(490)
Payments of tax withholdings related to vesting of employee stock awards and exercise of employee stock options (in shares)	(14)
Balance at Sep. 30, 2021	$ 141	$ 1,942,348	$ (27,437)	$ (1,384,516)	$ 530,536	$ 0	$ 530,536
Balance (in shares) at Sep. 30, 2021	141,184