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	84,443	84,443	0	84,443
Change in unrealized gains, net of tax	0	0	4,324	0	4,324	0	4,324
Foreign currency translation	0	0	84	0	84	0	84
Issuance of common stock in connection with employee stock plans	$ 2	67,057	0	0	67,059	0	67,059
Issuance of common stock in connection with employee stock plans (in shares)	1,825
Stock-based compensation expense	$ 0	45,505	0	0	45,505	0	45,505
Payments of tax withholdings related to vesting of employee stock awards and exercise of employee stock options	$ 0	(7,597)	0	0	(7,597)	0	(7,597)
Payments of tax withholdings related to vesting of employee stock awards and exercise of employee stock options (in shares)	(130)
Noncontrolling interest in Akcea Therapeutics, Inc.	$ 0	(34,246)	0	0	(34,246)	40,688	6,442
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
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	(48,226)	(48,226)	0	(48,226)
Change in unrealized gains, net of tax	0	0	(1,954)	0	(1,954)	0	(1,954)
Foreign currency translation	0	0	9	0	9	0	9
Issuance of common stock in connection with employee stock plans	$ 0	7,652	0	0	7,652	0	7,652
Issuance of common stock in connection with employee stock plans (in shares)	606
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	40,790	0	0	40,790	0	40,790
Payments of tax withholdings related to vesting of employee stock awards and exercise of employee stock options	$ 0	(11,603)	0	0	(11,603)	0	(11,603)
Payments of tax withholdings related to vesting of employee stock awards and exercise of employee stock options (in shares)	(186)
Noncontrolling interest in Akcea Therapeutics, Inc.	$ 0	(6,973)	0	0	(6,973)	(3,281)	(10,254)
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