CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) shares in Thousands, $ in Thousands	Common Stock [Member]	Additional Paid In Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Accumulated Deficit [Member]	Total Ionis Stockholders' Equity [Member]	Noncontrolling Interest in Akcea Therapeutics, Inc. [Member]	Total
Balance at Dec. 31, 2014	$ 118	$ 1,224,509	$ 39,747	$ (1,006,594)	$ 257,780	$ 0	$ 257,780
Balance (in shares) at Dec. 31, 2014	118,443
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	$ 0	0	0	(88,278)	(88,278)	0	(88,278)
Change in unrealized gains (losses), net of tax	0	0	(53,312)	0	(53,312)	0	(53,312)
Foreign currency translation							0
Issuance of common stock in connection with employee stock plans	$ 2	24,888	0	0	24,890	0	24,890
Issuance of common stock in connection with employee stock plans (in shares)	1,908
Stock-based compensation expense	$ 0	59,314	0	0	59,314	0	59,314
Noncontrolling interest in Akcea Therapetuics, Inc.							0
Excess tax benefits from stock-based compensation awards	0	396	0	0	396	0	396
Balance at Dec. 31, 2015	$ 120	1,309,107	(13,565)	(1,094,872)	200,790	0	200,790
Balance (in shares) at Dec. 31, 2015	120,351
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	$ 0	0	0	(86,556)	(86,556)	0	(86,556)
Change in unrealized gains (losses), net of tax	0	0	(16,772)	0	(16,772)	0	(16,772)
Foreign currency translation	0	0	(21)	0	(21)	0	(21)
Issuance of common stock in connection with employee stock plans	$ 2	13,706	0	0	13,708	0	13,708
Issuance of common stock in connection with employee stock plans (in shares)	1,285
2 3/4 percent convertible senior notes redemption, equity portion	$ 0	(128,888)	0	0	(128,888)	0	(128,888)
1 percent convertible senior notes, equity portion, net of issuance costs	0	43,335	0	0	43,335	0	43,335
Stock-based compensation expense	0	72,108	0	0	72,108	0	72,108
Noncontrolling interest in Akcea Therapetuics, Inc.							0
Excess tax benefits from stock-based compensation awards	0	1,861	0	0	1,861	0	1,861
Balance at Dec. 31, 2016	$ 122	1,311,229	(30,358)	(1,181,428)	99,565	0	99,565
Balance (in shares) at Dec. 31, 2016	121,636
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	$ 0	0	0	(5,970)	(5,970)	0	(5,970)
Change in unrealized gains (losses), net of tax	0	0	(1,334)	0	(1,334)	0	(1,334)
Foreign currency translation	0	0	(67)	0	(67)	0	(67)
Novartis stock purchase	$ 2	71,737	0	0	71,739	0	71,739
Novartis stock purchase (in shares)	1,631
Issuance of common stock in connection with employee stock plans	$ 1	22,931	0	0	22,932	0	22,932
Issuance of common stock in connection with employee stock plans (in shares)	1,709
Stock-based compensation expense	$ 0	85,975	0	0	85,975	0	85,975
Issuance of Akcea common stock in conjunction with intial public offering	$ 0	157,270	0	0	157,270	0	157,270
Issuance of Akcea common stock in conjunction with intial public offering (in shares)	0
Noncontrolling interest in Akcea Therapeutics, Inc. in conjunction with initial public offering	$ 0	(90,351)	0	0	(90,351)	90,381	30
Noncontrolling interest in Akcea Therapetuics, Inc.	0	(8,887)	0	0	(8,887)	(2,534)	(11,421)
Balance at Dec. 31, 2017	$ 125	$ 1,549,904	$ (31,759)	$ (1,187,398)	$ 330,872	$ 87,847	$ 418,719
Balance (in shares) at Dec. 31, 2017	124,976