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Segment Information and Concentration of Business Risk, Segment Information (Details) $ in Thousands		3 Months Ended		7 Months Ended
	Jul. 19, 2017	Mar. 31, 2018 USD ($) Segment	Mar. 31, 2017 USD ($)	Jul. 18, 2017	Apr. 30, 2018	Dec. 31, 2017 USD ($)
Segment Information [Abstract]
Number of reportable segments \| Segment		2
Segment Reporting Information [Line Items]
Revenue		$ 144,419	$ 115,800
Total operating expenses		147,720	96,315
Income (loss) from operations		(3,301)	19,485
Total assets as of current period		1,299,560				$ 1,322,774
Commercial Revenue [Member]
Segment Reporting Information [Line Items]
Revenue		42,023	7,801
SPINRAZA Royalties [Member]
Segment Reporting Information [Line Items]
Revenue		41,081	5,211
Licensing and Other Royalty Revenue [Member]
Segment Reporting Information [Line Items]
Revenue		942	2,590
R&D Revenue Under Collaborative Agreements [Member]
Segment Reporting Information [Line Items]
Revenue		$ 102,396	$ 107,999
Akcea [Member]
Segment Information [Abstract]
Percentage ownership before IPO			100.00%	100.00%
Percentage ownership after IPO	68.00%
Percentage ownership		68.00%
Akcea [Member] \| Subsequent Event [Member]
Segment Information [Abstract]
Percentage ownership					75.00%
Operating Segments [Member] \| Ionis Core [Member]
Segment Reporting Information [Line Items]
Revenue		$ 132,540	$ 161,183
Total operating expenses		105,544	78,352
Income (loss) from operations		26,996	82,831
Total assets as of current period		1,349,044				1,342,578
Operating Segments [Member] \| Ionis Core [Member] \| Commercial Revenue [Member]
Segment Reporting Information [Line Items]
Revenue		42,023	7,801
Operating Segments [Member] \| Ionis Core [Member] \| SPINRAZA Royalties [Member]
Segment Reporting Information [Line Items]
Revenue		41,081	5,211
Operating Segments [Member] \| Ionis Core [Member] \| Licensing and Other Royalty Revenue [Member]
Segment Reporting Information [Line Items]
Revenue		942	2,590
Operating Segments [Member] \| Ionis Core [Member] \| R&D Revenue Under Collaborative Agreements [Member]
Segment Reporting Information [Line Items]
Revenue		90,517	153,382
Operating Segments [Member] \| Akcea Therapeutics [Member]
Segment Reporting Information [Line Items]
Revenue		17,108	6,094
Total operating expenses		47,435	69,470
Income (loss) from operations		(30,327)	(63,376)
Total assets as of current period		252,466				268,804
Operating Segments [Member] \| Akcea Therapeutics [Member] \| Commercial Revenue [Member]
Segment Reporting Information [Line Items]
Revenue		0	0
Operating Segments [Member] \| Akcea Therapeutics [Member] \| SPINRAZA Royalties [Member]
Segment Reporting Information [Line Items]
Revenue		0	0
Operating Segments [Member] \| Akcea Therapeutics [Member] \| Licensing and Other Royalty Revenue [Member]
Segment Reporting Information [Line Items]
Revenue		0	0
Operating Segments [Member] \| Akcea Therapeutics [Member] \| R&D Revenue Under Collaborative Agreements [Member]
Segment Reporting Information [Line Items]
Revenue		17,108	6,094
Elimination of Intercompany Activity [Member]
Segment Reporting Information [Line Items]
Revenue		(5,229)	(51,477)
Total operating expenses		(5,259)	(51,507)
Income (loss) from operations		30	30
Total assets as of current period		(301,950)				$ (288,608)
Elimination of Intercompany Activity [Member] \| Commercial Revenue [Member]
Segment Reporting Information [Line Items]
Revenue		0	0
Elimination of Intercompany Activity [Member] \| SPINRAZA Royalties [Member]
Segment Reporting Information [Line Items]
Revenue		0	0
Elimination of Intercompany Activity [Member] \| Licensing and Other Royalty Revenue [Member]
Segment Reporting Information [Line Items]
Revenue		0	0
Elimination of Intercompany Activity [Member] \| R&D Revenue Under Collaborative Agreements [Member]
Segment Reporting Information [Line Items]
Revenue		$ (5,229)	$ (51,477)