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	(65,264)	(65,264)	0	(65,264)
Change in unrealized gains (losses), net of tax	0	0	9,251	0	9,251	0	9,251
Foreign currency translation	0	0	82	0	82	0	82
Issuance of common stock in connection with employee stock plans	$ 0	16,451	0	0	16,451	0	16,451
Issuance of common stock in connection with employee stock plans (in shares)	821
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	89,233	0	0	89,233	0	89,233
Payments of tax withholdings related to vesting of employee stock awards and exercise of employee stock options	$ 0	(11,970)	0	0	(11,970)	0	(11,970)
Payments of tax withholdings related to vesting of employee stock awards and exercise of employee stock options (in shares)	(194)
Noncontrolling interest in Akcea Therapeutics, Inc.	$ 0	(25,862)	(483)	0	(26,345)	4,167	(22,178)
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
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
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	$ 0	0	0	(25,636)	(25,636)	0	(25,636)
Change in unrealized gains (losses), net of tax	0	0	11,204	0	11,204	0	11,204
Foreign currency translation	0	0	74	0	74	0	74
Issuance of common stock in connection with employee stock plans	$ 0	8,800	0	0	8,800	0	8,800
Issuance of common stock in connection with employee stock plans (in shares)	214
Stock-based compensation expense	$ 0	48,442	0	0	48,442	0	48,442
Payments of tax withholdings related to vesting of employee stock awards and exercise of employee stock options	$ 0	(367)	0	0	(367)	0	(367)
Payments of tax withholdings related to vesting of employee stock awards and exercise of employee stock options (in shares)	(7)
Noncontrolling interest in Akcea Therapeutics, Inc.	$ 0	(18,889)	(483)	0	(19,372)	7,448	(11,924)
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
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	(170,744)	(170,744)	0	(170,744)
Change in unrealized gains (losses), net of tax	0	0	(4,703)	0	(4,703)	0	(4,703)
Foreign currency translation	0	0	(22)	0	(22)	0	(22)
Issuance of common stock in connection with employee stock plans	$ 1	9,641	0	0	9,642	0	9,642
Issuance of common stock in connection with employee stock plans (in shares)	917
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	67,882	0	0	67,882	0	67,882
Payments of tax withholdings related to vesting of employee stock awards and exercise of employee stock options	$ 0	(15,795)	0	0	(15,795)	0	(15,795)
Payments of tax withholdings related to vesting of employee stock awards and exercise of employee stock options (in shares)	(261)
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
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
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	$ 0	0	0	(80,875)	(80,875)	0	(80,875)
Change in unrealized gains (losses), net of tax	0	0	(1,697)	0	(1,697)	0	(1,697)
Foreign currency translation	0	0	104	0	104	0	104
Issuance of common stock in connection with employee stock plans	$ 0	1,882	0	0	1,882	0	1,882
Issuance of common stock in connection with employee stock plans (in shares)	108
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	30,022	0	0	30,022	0	30,022
Payments of tax withholdings related to vesting of employee stock awards and exercise of employee stock options	$ 0	(458)	0	0	(458)	0	(458)
Payments of tax withholdings related to vesting of employee stock awards and exercise of employee stock options (in shares)	(10)
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