CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS) [Abstract]
Net income (loss)	$ 303,262	$ 214,985	$ (10,783)
Unrealized gains (losses) on investments, net of tax	6,633	(280)	(960)
Reclassification adjustment for realized gains included in net income (loss)	0	0	(374)
Currency translation adjustment	93	23	(67)
Comprehensive income (loss)	309,988	214,728	(12,184)
Comprehensive income (loss) attributable to noncontrolling interest in Akcea Therapeutics, Inc.	9,118	(58,781)	(11,224)
Comprehensive income (loss) attributable to Ionis Pharmaceuticals, Inc. common stockholders	$ 300,870	$ 273,509	$ (960)