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]
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	$ 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 Therapeutics, 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	$ 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 Therapeutics, 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
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	$ 0			0			0			294,146			294,146			0			294,146
Change in unrealized gains (losses), net of tax	0			0			6,633			0			6,633			0			6,633
Foreign currency translation	0			0			93			0			93			0			93
Issuance of common stock in connection with employee stock plans	$ 3			119,654			0			0			119,657			0			119,657
Issuance of common stock in connection with employee stock plans (in shares)	3,100
1 percent convertible senior notes retirement, equity portion, net of tax	$ 0			(77,331)			0			0			(77,331)			0			(77,331)
0.125 percent convertible senior notes, equity portion, net of issuance costs and tax	0			81,877			0			0			81,877			0			81,877
Issuance of warrants	0			56,110			0			0			56,110			0			56,110
Purchase of note hedges, net of tax	0			(85,860)			0			0			(85,860)			0			(85,860)
Repurchases and retirements of common stock	$ (1)			0			0			(34,387)			(34,388)			0			(34,388)
Repurchases and retirements of common stock (in shares)	(535)
Stock-based compensation expense	$ 0			146,574			0			0			146,574			0			146,574
Payments of tax withholdings related to vesting of employee stock awards and exercise of employee stock options	$ 0			(19,242)			0			0			(19,242)			0			(19,242)
Payments of tax withholdings related to vesting of employee stock awards and exercise of employee stock options (in shares)	(154)
Noncontrolling interest in Akcea Therapeutics, Inc.	$ 0			(65,254)			0			0			(65,254)			74,372			9,118
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