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	83,567	83,567	0	83,567
Change in unrealized gains, net of tax	0	0	7,775	0	7,775	0	7,775
Foreign currency translation	0	0	(11)	0	(11)	0	(11)
Issuance of common stock in connection with employee stock plans	$ 2	102,002	0	0	102,004	0	102,004
Issuance of common stock in connection with employee stock plans (in shares)	2,600
Stock-based compensation expense	$ 0	87,437	0	0	87,437	0	87,437
Payments of tax withholdings related to vesting of employee stock awards and exercise of employee stock options	$ 0	(8,034)	0	0	(8,034)	0	(8,034)
Payments of tax withholdings related to vesting of employee stock awards and exercise of employee stock options (in shares)	(136)
Noncontrolling interest in Akcea Therapeutics, Inc.	$ 0	(51,433)	0	0	(51,433)	48,737	(2,696)
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
Balance at Mar. 31, 2019	$ 140	2,117,969	(27,608)	(882,850)	1,207,651	179,769	1,387,420
Balance (in shares) at Mar. 31, 2019	139,624
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	$ 0	0	0	(876)	(876)	0	(876)
Change in unrealized gains, net of tax	0	0	3,452	0	3,452	0	3,452
Foreign currency translation	0	0	(96)	0	(96)	0	(96)
Issuance of common stock in connection with employee stock plans	$ 0	34,943	0	0	34,943	0	34,943
Issuance of common stock in connection with employee stock plans (in shares)	774
Stock-based compensation expense	$ 0	41,933	0	0	41,933	0	41,933
Payments of tax withholdings related to vesting of employee stock awards and exercise of employee stock options	$ 0	(438)	0	0	(438)	0	(438)
Payments of tax withholdings related to vesting of employee stock awards and exercise of employee stock options (in shares)	(5)
Noncontrolling interest in Akcea Therapeutics, Inc.	$ 0	(17,185)	0	0	(17,185)	8,049	(9,136)
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
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	(80,071)	(80,071)	0	(80,071)
Change in unrealized gains, 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	2,271,630	(16,440)	(878,154)	1,377,175	217,620	1,594,795
Balance (in shares) at Jun. 30, 2020	139,489
Balance at Mar. 31, 2020	$ 139	2,233,644	(27,235)	(846,309)	1,360,239	210,172	1,570,411
Balance (in shares) at Mar. 31, 2020	139,282
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	$ 0	0	0	(31,845)	(31,845)	0	(31,845)
Change in unrealized gains, 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	$ 2,271,630	$ (16,440)	$ (878,154)	$ 1,377,175	$ 217,620	$ 1,594,795
Balance (in shares) at Jun. 30, 2020	139,489