CONDENSED CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (Unaudited) - 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, 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	109,730	109,730	0	109,730
Change in unrealized gains (losses), net of tax	0	0	6,666	0	6,666	0	6,666
Foreign currency translation	0	0	(25)	0	(25)	0	(25)
Issuance of common stock in connection with employee stock plans	$ 3	112,132	0	0	112,135	0	112,135
Issuance of common stock in connection with employee stock plans (in shares)	2,855
Stock-based compensation expense	$ 0	111,564	0	0	111,564	0	111,564
Payments of tax withholdings related to vesting of employee stock awards and exercise of employee stock options	$ 0	(18,841)	0	0	(18,841)	0	(18,841)
Payments of tax withholdings related to vesting of employee stock awards and exercise of employee stock options (in shares)	(147)
Noncontrolling interest in Akcea Therapeutics, Inc.	$ 0	(33,015)	0	0	(33,015)	22,592	(10,423)
Balance at Sep. 30, 2019	$ 141	2,219,090	(25,375)	(857,563)	1,336,293	161,673	1,497,966
Balance (in shares) at Sep. 30, 2019	140,637
Balance at Jun. 30, 2019	$ 140	2,177,222	(24,252)	(883,726)	1,269,384	187,818	1,457,202
Balance (in shares) at Jun. 30, 2019	140,393
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	$ 0	0	0	26,163	26,163	0	26,163
Change in unrealized gains (losses), net of tax	0	0	(1,110)	0	(1,110)	0	(1,110)
Foreign currency translation	0	0	(13)	0	(13)	0	(13)
Issuance of common stock in connection with employee stock plans	$ 1	10,130	0	0	10,131	0	10,131
Issuance of common stock in connection with employee stock plans (in shares)	255
Stock-based compensation expense	$ 0	24,126	0	0	24,126	0	24,126
Payments of tax withholdings related to vesting of employee stock awards and exercise of employee stock options	$ 0	(10,804)	0	0	(10,804)	0	(10,804)
Payments of tax withholdings related to vesting of employee stock awards and exercise of employee stock options (in shares)	(11)
Noncontrolling interest in Akcea Therapeutics, Inc.	$ 0	18,416	0	0	18,416	(26,145)	(7,729)
Balance at Sep. 30, 2019	$ 141	2,219,090	(25,375)	(857,563)	1,336,293	161,673	1,497,966
Balance (in shares) at Sep. 30, 2019	140,637
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	(111,015)	(111,015)	0	(111,015)
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	2,311,679	(19,572)	(909,098)	1,383,149	223,235	1,606,384
Balance (in shares) at Sep. 30, 2020	139,794
Balance at Jun. 30, 2020	$ 139	2,271,630	(16,440)	(878,154)	1,377,175	217,620	1,594,795
Balance (in shares) at Jun. 30, 2020	139,489
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	$ 0	0	0	(30,944)	(30,944)	0	(30,944)
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	$ 2,311,679	$ (19,572)	$ (909,098)	$ 1,383,149	$ 223,235	$ 1,606,384
Balance (in shares) at Sep. 30, 2020	139,794