Segment Information and Concentration of Business Risk, Segment Information (Details) $ in Thousands	1 Months Ended	3 Months Ended								12 Months Ended
	May 31, 2015 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($) Segment	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Jul. 31, 2017
Segment Information and Concentration of Business Risk [Abstract]
Number of reportable segments | Segment										2
Segment Information [Abstract]
Revenue	$ 91,200									$ 599,674	$ 514,179	$ 372,776
Total operating expenses		$ 181,331	$ 163,967	$ 168,028	$ 147,720	$ 173,992	$ 107,002	$ 105,823	$ 96,315	661,046	483,132	392,936
Income (loss) from operations		10,782	$ (18,572)	$ (50,281)	$ (3,301)	(6,200)	$ 11,312	$ 6,450	$ 19,485	(61,372)	31,047	(20,160)
Total assets as of current period		$ 2,667,784				1,322,774				2,667,784	1,322,774
Commercial Revenue [Member]
Segment Information [Abstract]
Revenue										254,922	120,014	22,767
SPINRAZA Royalties [Member]
Segment Information [Abstract]
Revenue										237,930	112,540	883
TEGSEDI Product Sales, Net [Member]
Segment Information [Abstract]
Revenue										2,237	0	0
Licensing and Other Royalty Revenue [Member]
Segment Information [Abstract]
Revenue										14,755	7,474	21,884
R&D Revenue Under Collaborative Agreements [Member]
Segment Information [Abstract]
Revenue										$ 344,752	394,165	350,009
Akcea [Member]
Segment Information and Concentration of Business Risk [Abstract]
Percentage ownership		75.00%			68.00%					75.00%			68.00%
Operating Segments [Member] | Ionis Core [Member]
Segment Information [Abstract]
Revenue										$ 641,944	525,185	385,424
Total operating expenses										380,212	373,788	322,192
Income (loss) from operations										261,732	151,397	63,232
Total assets as of current period		$ 2,975,491				1,342,578				2,975,491	1,342,578
Operating Segments [Member] | Ionis Core [Member] | Commercial Revenue [Member]
Segment Information [Abstract]
Revenue										240,685	120,014	22,767
Operating Segments [Member] | Ionis Core [Member] | SPINRAZA Royalties [Member]
Segment Information [Abstract]
Revenue										237,930	112,540	883
Operating Segments [Member] | Ionis Core [Member] | TEGSEDI Product Sales, Net [Member]
Segment Information [Abstract]
Revenue										0
Operating Segments [Member] | Ionis Core [Member] | Licensing and Other Royalty Revenue [Member]
Segment Information [Abstract]
Revenue										2,755	7,474	21,884
Operating Segments [Member] | Ionis Core [Member] | R&D Revenue Under Collaborative Agreements [Member]
Segment Information [Abstract]
Revenue										401,259	405,171	362,657
Operating Segments [Member] | Akcea Therapeutics [Member]
Segment Information [Abstract]
Revenue										64,867	43,401	0
Total operating expenses										295,683	163,871	83,512
Income (loss) from operations										(230,816)	(120,470)	(83,512)
Total assets as of current period		365,261				268,804				365,261	268,804
Operating Segments [Member] | Akcea Therapeutics [Member] | Commercial Revenue [Member]
Segment Information [Abstract]
Revenue										14,237	0	0
Operating Segments [Member] | Akcea Therapeutics [Member] | SPINRAZA Royalties [Member]
Segment Information [Abstract]
Revenue										0	0	0
Operating Segments [Member] | Akcea Therapeutics [Member] | TEGSEDI Product Sales, Net [Member]
Segment Information [Abstract]
Revenue										2,237
Operating Segments [Member] | Akcea Therapeutics [Member] | Licensing and Other Royalty Revenue [Member]
Segment Information [Abstract]
Revenue										12,000	0	0
Operating Segments [Member] | Akcea Therapeutics [Member] | R&D Revenue Under Collaborative Agreements [Member]
Segment Information [Abstract]
Revenue										50,630	43,401	0
Elimination of Intercompany Activity [Member]
Segment Information [Abstract]
Revenue										(107,137)	(54,407)	(12,648)
Total operating expenses										(14,849)	(54,527)	(12,768)
Income (loss) from operations										(92,288)	120	120
Total assets as of current period		$ (672,968)				$ (288,608)				(672,968)	(288,608)
Elimination of Intercompany Activity [Member] | Commercial Revenue [Member]
Segment Information [Abstract]
Revenue										0	0	0
Elimination of Intercompany Activity [Member] | SPINRAZA Royalties [Member]
Segment Information [Abstract]
Revenue										0	0	0
Elimination of Intercompany Activity [Member] | TEGSEDI Product Sales, Net [Member]
Segment Information [Abstract]
Revenue										0
Elimination of Intercompany Activity [Member] | Licensing and Other Royalty Revenue [Member]
Segment Information [Abstract]
Revenue										0	0	0
Elimination of Intercompany Activity [Member] | R&D Revenue Under Collaborative Agreements [Member]
Segment Information [Abstract]
Revenue										$ (107,137)	$ (54,407)	$ (12,648)