Significant Accounting Policies, Convertible Debt (Details) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2021 USD ($) Note $ / shares	Mar. 31, 2020 USD ($) $ / shares	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Convertible Debt [Abstract]
Number of outstanding convertible notes | Note	2
Condensed Consolidated Balance Sheet [Abstract]
1 percent convertible senior notes, net	$ 61,816		$ 308,809
Additional paid-in capital	1,925,801		1,895,519
Accumulated deficit	(1,221,175)		(1,131,306)
Condensed Consolidated Statement of Operations [Abstract]
Interest expense	(2,414)	$ (2,207)
Loss before income tax (expense) benefit	(89,739)	(52,954)
Income tax (expense) benefit	(130)	3,072
Net loss	(89,869)	(49,882)
Net loss attributable to Ionis Pharmaceuticals, Inc. common stockholders	$ (89,869)	$ (39,628)
Basic net loss per share (in dollars per share) | $ / shares	$ (0.64)	$ (0.28)
Diluted net loss per share (in dollars per share) | $ / shares	$ (0.64)	$ (0.28)
Condensed Consolidated Statements of Stockholders' Equity [Abstract]
Balance	$ 680,564	$ 1,471,920	743,282	$ 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
Condensed Consolidated Statements of Stockholders' Equity [Abstract]
Balance	1,925,801	2,015,516	1,895,519	1,985,650
Accumulated Deficit [Member]
Condensed Consolidated Statement of Operations [Abstract]
Net loss attributable to Ionis Pharmaceuticals, Inc. common stockholders	(89,869)	(39,628)
Condensed Consolidated Statements of Stockholders' Equity [Abstract]
Balance	(1,221,175)	(726,672)	(1,131,306)	(596,495)
Total Ionis Stockholders' Equity [Member]
Condensed Consolidated Statement of Operations [Abstract]
Net loss attributable to Ionis Pharmaceuticals, Inc. common stockholders	(89,869)	(39,628)
Condensed Consolidated Statements of Stockholders' Equity [Abstract]
Balance	$ 680,564	1,261,748	$ 743,282	1,364,005
0.125 Percent Convertible Senior Notes [Member]
Convertible Debt [Abstract]
Interest rate on convertible senior notes	0.125%		0.125%
Condensed Consolidated Balance Sheet [Abstract]
Convertible senior notes	$ 540,679		$ 540,136
Condensed Consolidated Statement of Operations [Abstract]
Effective interest rate	0.50%
1 Percent Convertible Senior Notes [Member]
Convertible Debt [Abstract]
Interest rate on convertible senior notes	1.00%		1.00%
Condensed Consolidated Balance Sheet [Abstract]
Convertible senior notes	$ 247,292		$ 0
Condensed Consolidated Statement of Operations [Abstract]
Effective interest rate	1.40%
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		(10,990)
Loss before income tax (expense) benefit		(61,737)
Income tax (expense) benefit		3,257
Net loss		(58,480)
Net loss attributable to Ionis Pharmaceuticals, Inc. common stockholders		$ (48,226)
Basic net loss per share (in dollars per share) | $ / shares		$ (0.35)
Diluted net loss per share (in dollars per share) | $ / shares		$ (0.35)
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		$ 8,783
Loss before income tax (expense) benefit		8,783
Income tax (expense) benefit		(185)
Net loss		8,598
Net loss attributable to Ionis Pharmaceuticals, Inc. common stockholders		$ 8,598
Basic net loss per share (in dollars per share) | $ / shares		$ 0.07
Diluted net loss per share (in dollars per share) | $ / shares		$ 0.07
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