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, 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	(1,420)	(1,420)	0	(1,420)
Change in unrealized gains (losses), net of tax	0	0	(1,530)	0	(1,530)	0	(1,530)
Foreign currency translation	0	0	55	0	55	0	55
Issuance of common stock in connection with employee stock plans	$ 0	5,664	0	0	5,664	0	5,664
Issuance of common stock in connection with employee stock plans (in shares)	473
Stock-based compensation expense	$ 0	28,451	0	0	28,451	0	28,451
Noncontrolling interest in Akcea Therapetuics, Inc.	0	(10,842)	0	0	(10,842)	1,443	(9,399)
Balance at Mar. 31, 2018	$ 125	1,576,954	(33,234)	(1,242,454)	301,391	85,710	387,101
Balance (in shares) at Mar. 31, 2018	125,449
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 (losses), 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 share-based awards	$ 0	(7,597)	0	0	(7,597)	0	(7,597)
Payments of tax withholdings related to vesting of share-based awards (in shares)	(130)
Noncontrolling interest in Akcea Therapetuics, 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