Significant Accounting Policies, Convertible Debt (Details) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021 USD ($) Note $ / shares	Jun. 30, 2020 USD ($) $ / shares	Jun. 30, 2021 USD ($) Note $ / shares	Jun. 30, 2020 USD ($) $ / shares	Apr. 30, 2021 USD ($)	Mar. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Convertible Debt [Abstract]
Number of outstanding convertible notes | Note	2		2
Condensed Consolidated Balance Sheet [Abstract]
1 percent convertible senior notes, net	$ 61,876		$ 61,876				$ 308,809
Additional paid-in capital	1,910,379		1,910,379				1,895,519
Accumulated deficit	(1,302,050)		(1,302,050)				(1,131,306)
Condensed Consolidated Statement of Operations [Abstract]
Interest expense	(2,357)	$ (2,441)	(4,771)	$ (4,648)
Loss before income tax benefit (expense)	(80,548)	(35,476)	(170,287)	(88,430)
Income tax benefit (expense)	(327)	(2,084)	(457)	988
Net loss	(80,875)	(37,560)	(170,744)	(87,442)
Net loss attributable to Ionis Pharmaceuticals, Inc. common stockholders	$ (80,875)	$ (25,636)	$ (170,744)	$ (65,264)
Basic net loss per share (in dollars per share) | $ / shares	$ (0.57)	$ (0.18)	$ (1.21)	$ (0.47)
Diluted net loss per share (in dollars per share) | $ / shares	$ (0.57)	$ (0.18)	$ (1.21)	$ (0.47)
Condensed Consolidated Statements of Stockholders' Equity [Abstract]
Balance	$ 582,674	$ 1,502,513	$ 582,674	$ 1,502,513		$ 680,564	743,282	$ 1,471,920	$ 1,577,458
Additional Paid In Capital [Member]
Condensed Consolidated Statement of Operations [Abstract]
Net loss attributable to Ionis Pharmaceuticals, Inc. common stockholders	0	0	0	0
Condensed Consolidated Statements of Stockholders' Equity [Abstract]
Balance	1,910,379	2,053,502	1,910,379	2,053,502		1,925,801	1,895,519	2,015,516	1,985,650
Accumulated Deficit [Member]
Condensed Consolidated Statement of Operations [Abstract]
Net loss attributable to Ionis Pharmaceuticals, Inc. common stockholders	(80,875)	(25,636)	(170,744)	(65,264)
Condensed Consolidated Statements of Stockholders' Equity [Abstract]
Balance	(1,302,050)	(752,308)	(1,302,050)	(752,308)		(1,221,175)	(1,131,306)	(726,672)	(596,495)
Total Ionis Stockholders' Equity [Member]
Condensed Consolidated Statement of Operations [Abstract]
Net loss attributable to Ionis Pharmaceuticals, Inc. common stockholders	(80,875)	(25,636)	(170,744)	(65,264)
Condensed Consolidated Statements of Stockholders' Equity [Abstract]
Balance	$ 582,674	$ 1,284,893	$ 582,674	$ 1,284,893		$ 680,564	$ 743,282	$ 1,261,748	1,364,005
0.125 Percent Convertible Senior Notes [Member]
Convertible Debt [Abstract]
Interest rate on convertible senior notes	0.125%	0.125%	0.125%	0.125%			0.125%
Condensed Consolidated Balance Sheet [Abstract]
Convertible senior notes	$ 541,223		$ 541,223				$ 540,136
Condensed Consolidated Statement of Operations [Abstract]
Effective interest rate	0.50%		0.50%
1 Percent Convertible Senior Notes [Member]
Convertible Debt [Abstract]
Interest rate on convertible senior notes	1.00%	1.00%	1.00%	1.00%			1.00%
Condensed Consolidated Statement of Operations [Abstract]
Effective interest rate	1.40%		1.40%
0 Percent Convertible Senior Notes [Member]
Convertible Debt [Abstract]
Interest rate on convertible senior notes	0.00%	0.00%	0.00%	0.00%
Face amount of offering					$ 632,500
Condensed Consolidated Balance Sheet [Abstract]
Convertible senior notes	$ 617,399		$ 617,399				$ 0
Condensed Consolidated Statement of Operations [Abstract]
Effective interest rate	0.50%		0.50%
As Previously Reported [Member]
Condensed Consolidated Balance Sheet [Abstract]
1 percent convertible senior notes, net							293,161
Additional paid-in capital							2,113,646
Accumulated deficit							(1,249,368)
Condensed Consolidated Statement of Operations [Abstract]
Interest expense		$ (11,173)		$ (22,163)
Loss before income tax benefit (expense)		(44,208)		(105,945)
Income tax benefit (expense)		439		3,696
Net loss		(43,769)		(102,249)
Net loss attributable to Ionis Pharmaceuticals, Inc. common stockholders		$ (31,845)		$ (80,071)
Basic net loss per share (in dollars per share) | $ / shares		$ (0.23)		$ (0.58)
Diluted net loss per share (in dollars per share) | $ / shares		$ (0.23)		$ (0.58)
Condensed Consolidated Statements of Stockholders' Equity [Abstract]
Balance							843,347		1,684,547
As Previously Reported [Member] | Additional Paid In Capital [Member]
Condensed Consolidated Statements of Stockholders' Equity [Abstract]
Balance							2,113,646		2,203,778
As Previously Reported [Member] | Accumulated Deficit [Member]
Condensed Consolidated Statements of Stockholders' Equity [Abstract]
Balance							(1,249,368)		(707,534)
As Previously Reported [Member] | 0.125 Percent Convertible Senior Notes [Member]
Condensed Consolidated Balance Sheet [Abstract]
Convertible senior notes							455,719
Adjustment [Member] | ASU 2020-06 [Member]
Condensed Consolidated Balance Sheet [Abstract]
1 percent convertible senior notes, net							15,648
Additional paid-in capital							(218,127)
Accumulated deficit							118,062
Net deferred tax assets							0
Condensed Consolidated Statement of Operations [Abstract]
Interest expense		$ 8,732		$ 17,515
Loss before income tax benefit (expense)		8,732		17,515
Income tax benefit (expense)		(2,523)		(2,708)
Net loss		6,209		14,807
Net loss attributable to Ionis Pharmaceuticals, Inc. common stockholders		$ 6,209		$ 14,807
Basic net loss per share (in dollars per share) | $ / shares		$ 0.05		$ 0.11
Diluted net loss per share (in dollars per share) | $ / shares		$ 0.05		$ 0.11
Condensed Consolidated Statements of Stockholders' Equity [Abstract]
Balance							(100,065)		(107,089)
Adjustment [Member] | ASU 2020-06 [Member] | Additional Paid In Capital [Member]
Condensed Consolidated Statements of Stockholders' Equity [Abstract]
Balance							(218,127)		(218,128)
Adjustment [Member] | ASU 2020-06 [Member] | Accumulated Deficit [Member]
Condensed Consolidated Statements of Stockholders' Equity [Abstract]
Balance							118,062		$ 111,039
Adjustment [Member] | ASU 2020-06 [Member] | 0.125 Percent Convertible Senior Notes [Member]
Condensed Consolidated Balance Sheet [Abstract]
Convertible senior notes							$ 84,417