CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE LOSS - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE LOSS [Abstract]
Net loss	$ (89,869)	$ (49,882)
Unrealized losses on debt securities, net of tax	(3,006)	(1,954)
Currency translation adjustment	(126)	9
Comprehensive loss	(93,001)	(51,827)
Comprehensive loss attributable to noncontrolling interest in Akcea Therapeutics, Inc.	0	(10,254)
Comprehensive loss attributable to Ionis Pharmaceuticals, Inc. common stockholders	$ (93,001)	$ (41,573)