CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) shares in Thousands, $ in Thousands	Common Stock [Member]	Common Stock [Member] Novartis [Member]	Common Stock [Member] Biogen Inc. [Member]	Additional Paid In Capital [Member]	Additional Paid In Capital [Member] Novartis [Member]	Additional Paid In Capital [Member] Biogen Inc. [Member]	Accumulated Other Comprehensive Loss [Member]	Accumulated Other Comprehensive Loss [Member] Novartis [Member]	Accumulated Other Comprehensive Loss [Member] Biogen Inc. [Member]	Accumulated Deficit [Member]	Accumulated Deficit [Member] Novartis [Member]	Accumulated Deficit [Member] Biogen Inc. [Member]	Total Ionis Stockholders' Equity [Member]	Total Ionis Stockholders' Equity [Member] Novartis [Member]	Total Ionis Stockholders' Equity [Member] Biogen Inc. [Member]	Noncontrolling Interest in Akcea Therapeutics, Inc. [Member]	Noncontrolling Interest in Akcea Therapeutics, Inc. [Member] Novartis [Member]	Noncontrolling Interest in Akcea Therapeutics, Inc. [Member] Biogen Inc. [Member]	Total	Novartis [Member]	Biogen Inc. [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative adjustment related to adopting Topic 606 revenue recognition guidance | Topic 606 [Member]	$ 0			$ 0			$ 0			$ (86,108)			$ (86,108)			$ 0			$ (86,108)
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 income (loss)	$ 0			0			0			(60,400)			(60,400)			0			(60,400)
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)			(1,241,380)			39,613			0			39,613
Balance (in shares) at Dec. 31, 2016	121,636
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	$ 0			0			0			346			346			0			346
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)
Stock purchase		$ 2			$ 71,737			$ 0			$ 0			$ 71,739			$ 0			$ 71,739
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 Therapeutics, Inc. common stock in conjunction with intial public offering	$ 0			157,270			0			0			157,270			0			157,270
Issuance of Akcea Therapeutics, Inc. 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			(5,110)			0			0			(5,110)			(6,114)			(11,224)
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			273,741			273,741			0			273,741
Change in unrealized gains (losses), net of tax	0			0			(280)			0			(280)			0			(280)
Foreign currency translation	0			0			23			0			23			0			23
Stock purchase			$ 11			$ 447,954			$ 0			$ 0			$ 447,965			$ 0			$ 447,965
Stock purchase (in shares)			11,502
Issuance of common stock in connection with employee stock plans	$ 2			27,898			0			0			27,900			0			27,900
Issuance of common stock in connection with employee stock plans (in shares)	1,451
Stock-based compensation expense	$ 0			131,312			0			0			131,312			0			131,312
Noncontrolling interest in Akcea Therapetuics, Inc.	0			(113,595)			0			0			(113,595)			54,814			(58,781)
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