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, 2017	$ 125	$ 1,553,681	$ (31,759)	$ (1,241,034)	$ 281,013	$ 84,267	$ 365,280
Balance (in shares) at Dec. 31, 2017	124,976
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	$ 0	0	0	273,741	273,741	0	273,741
Change in unrealized gains (losses), net of tax	0	0	(280)	0	(280)	0	(280)
Foreign currency translation	0	0	23	0	23	0	23
Biogen stock purchase	$ 11	447,954	0	0	447,965	0	447,965
Biogen Stock purchase (in shares)	11,502
Issuance of common stock in connection with employee stock plans	$ 2	27,898	0	0	27,900	0	27,900
Issuance of common stock in connection with employee stock plans (in shares)	1,451
Stock-based compensation expense	$ 0	131,312	0	0	131,312	0	131,312
Noncontrolling interest in Akcea Therapeutics, Inc.	0	(113,595)	0	0	(113,595)	54,814	(58,781)
Balance at Dec. 31, 2018	$ 138	2,047,250	(32,016)	(967,293)	1,048,079	139,081	1,187,160
Balance (in shares) at Dec. 31, 2018	137,929
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	$ 0	0	0	294,146	294,146	0	294,146
Change in unrealized gains (losses), net of tax	0	0	6,633	0	6,633	0	6,633
Foreign currency translation	0	0	93	0	93	0	93
Issuance of common stock in connection with employee stock plans	$ 3	119,654	0	0	119,657	0	119,657
Issuance of common stock in connection with employee stock plans (in shares)	3,100
1 percent convertible senior notes retirement, equity portion, net of tax	$ 0	(77,331)	0	0	(77,331)	0	(77,331)
0.125 percent convertible senior notes, equity portion, net of issuance costs and tax	0	81,877	0	0	81,877	0	81,877
Issuance of warrants	0	56,110	0	0	56,110	0	56,110
Purchase of note hedges, net of tax	0	(85,860)	0	0	(85,860)	0	(85,860)
Repurchases and retirements of common stock	$ (1)	0	0	(34,387)	(34,388)	0	(34,388)
Repurchases and retirements of common stock (in shares)	(535)
Stock-based compensation expense	$ 0	146,574	0	0	146,574	0	146,574
Payments of tax withholdings related to vesting of employee stock awards and exercise of employee stock options	$ 0	(19,242)	0	0	(19,242)	0	(19,242)
Payments of tax withholdings related to vesting of employee stock awards and exercise of employee stock options (in shares)	(154)
Noncontrolling interest in Akcea Therapeutics, Inc.	$ 0	(65,254)	0	0	(65,254)	74,372	9,118
Balance at Dec. 31, 2019	$ 140	2,203,778	(25,290)	(707,534)	1,471,094	213,453	1,684,547
Balance (in shares) at Dec. 31, 2019	140,340
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	$ 0	0	0	(451,286)	(451,286)	0	(451,286)
Change in unrealized gains (losses), net of tax	0	0	3,729	0	3,729	0	3,729
Foreign currency translation	0	0	617	0	617	0	617
Issuance of common stock in connection with employee stock plans	$ 1	52,033	0	0	52,034	0	52,034
Issuance of common stock in connection with employee stock plans (in shares)	1,721
Purchase of noncontrolling interest of Akcea Therapeutics, Inc., including cash payments for cancellation of Akcea Therapeutics, Inc. equity awards	$ 0	(324,022)	301	0	(323,721)	(220,965)	(544,686)
Repurchases and retirements of common stock	$ (1)	0	0	(90,548)	(90,549)	0	(90,549)
Repurchases and retirements of common stock (in shares)	(1,478)
Stock-based compensation expense	$ 0	230,117	0	0	230,117	0	230,117
Payments of tax withholdings related to vesting of employee stock awards and exercise of employee stock options	$ 0	(13,410)	0	0	(13,410)	0	(13,410)
Payments of tax withholdings related to vesting of employee stock awards and exercise of employee stock options (in shares)	(217)
Deferred tax liability adjustment due to purchase of noncontrolling interest of Akcea Therapeutics, Inc.	$ 0	7,714	0	0	7,714	0	7,714
Noncontrolling interest in Akcea Therapeutics, Inc.	0	(42,564)	(428)	0	(42,992)	7,512	(35,480)
Balance at Dec. 31, 2020	$ 140	$ 2,113,646	$ (21,071)	$ (1,249,368)	$ 843,347	$ 0	$ 843,347
Balance (in shares) at Dec. 31, 2020	140,366