The number of shares purchased in the subsidiary.

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The number of shares received from the subsidiary when TEGSEDI received marketing authorization in the European Union (EU).

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The number of shares received from the subsidiary for the license of TEGSEDI (inotersen) and AKCEA-TTR-L to the subsidiary.

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Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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The parent entity's interest in net assets of the subsidiary, expressed as a percentage.

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Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements.

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Generally recurring costs associated with normal operations except for the portion of these expenses which can be clearly related to production and included in cost of sales or services. Includes selling, general and administrative expense.

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No definition available.

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The net result for the period of deducting operating expenses from operating revenues.

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The cash outflow associated with the purchase of noncontrolling interest during the period.

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Amount, excluding tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value added and excise.

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Percentage of subsidiary's or equity investee's stock owned by parent company before stock transaction.

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Segment Information and Concentration of Business Risk, Segment Information (Details) $ in Thousands, shares in Millions		1 Months Ended	3 Months Ended			6 Months Ended		7 Months Ended
	Aug. 07, 2018 shares	Apr. 30, 2018 USD ($) shares	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017	Jun. 30, 2018 USD ($) Segment	Jun. 30, 2017 USD ($)	Jul. 31, 2017	Jul. 18, 2017	Mar. 31, 2018	Dec. 31, 2017 USD ($)
Segment Information and Concentration of Business Risk [Abstract]
Number of reportable segments \| Segment						2
Segment Information [Abstract]
Revenue			$ 117,747	$ 112,273		$ 262,165	$ 228,073
Total operating expenses			168,028	105,823		315,748	202,138
Income (loss) from operations			(50,281)	6,450		(53,583)	25,935
Total assets as of current period			2,247,315			2,247,315					$ 1,322,774
Commercial Revenue [Member]
Segment Information [Abstract]
Revenue			57,198	23,688		99,221	31,489
SPINRAZA Royalties [Member]
Segment Information [Abstract]
Revenue			56,653	22,366		97,734	27,577
Licensing and Other Royalties [Member]
Segment Information [Abstract]
Revenue			545	1,322		1,487	3,912
R&D Revenue Under Collaborative Agreements [Member]
Segment Information [Abstract]
Revenue			60,549	88,585		162,944	$ 196,584
Akcea [Member]
Segment Information [Abstract]
Percentage ownership before IPO					100.00%		100.00%	100.00%	100.00%
Percentage ownership		75.00%						68.00%		68.00%
Shares of Akcea stock received for license of TEGSEDI and AKCEA-TTR-L (in shares) \| shares		8.0
Additional shares of Akcea stock purchased (in shares) \| shares		10.7
Additional stock purchased in Akcea		$ 200,000
Akcea [Member] \| Subsequent Event [Member]
Segment Information [Abstract]
Additional shares of Akcea stock received when TEGSEDI received marketing authorization in the EU (in shares) \| shares	1.6
Operating Segments [Member] \| Ionis Core [Member]
Segment Information [Abstract]
Revenue			99,426	109,490		231,965	$ 270,673
Total operating expenses			85,875	83,381		191,419	161,733
Income (loss) from operations			13,551	26,109		40,546	108,940
Total assets as of current period			2,351,098			2,351,098					1,342,578
Operating Segments [Member] \| Ionis Core [Member] \| Commercial Revenue [Member]
Segment Information [Abstract]
Revenue			57,198	23,688		99,221	31,489
Operating Segments [Member] \| Ionis Core [Member] \| SPINRAZA Royalties [Member]
Segment Information [Abstract]
Revenue			56,653	22,366		97,734	27,577
Operating Segments [Member] \| Ionis Core [Member] \| Licensing and Other Royalties [Member]
Segment Information [Abstract]
Revenue			545	1,322		1,487	3,912
Operating Segments [Member] \| Ionis Core [Member] \| R&D Revenue Under Collaborative Agreements [Member]
Segment Information [Abstract]
Revenue			42,228	85,802		132,744	239,184
Operating Segments [Member] \| Akcea Therapeutics [Member]
Segment Information [Abstract]
Revenue			18,321	5,713		35,429	11,807
Total operating expenses			81,744	25,402		129,179	94,872
Income (loss) from operations			(63,423)	(19,689)		(93,750)	(83,065)
Total assets as of current period			395,005			395,005					268,804
Operating Segments [Member] \| Akcea Therapeutics [Member] \| Commercial Revenue [Member]
Segment Information [Abstract]
Revenue			0	0		0	0
Operating Segments [Member] \| Akcea Therapeutics [Member] \| SPINRAZA Royalties [Member]
Segment Information [Abstract]
Revenue			0	0		0	0
Operating Segments [Member] \| Akcea Therapeutics [Member] \| Licensing and Other Royalties [Member]
Segment Information [Abstract]
Revenue			0	0		0	0
Operating Segments [Member] \| Akcea Therapeutics [Member] \| R&D Revenue Under Collaborative Agreements [Member]
Segment Information [Abstract]
Revenue			18,321	5,713		35,429	11,807
Elimination of Intercompany Activity [Member]
Segment Information [Abstract]
Revenue			0	(2,930)		(5,229)	(54,407)
Total operating expenses			409	(2,960)		(4,850)	(54,467)
Income (loss) from operations			(409)	30		(379)	60
Total assets as of current period			(498,788)			(498,788)					$ (288,608)
Elimination of Intercompany Activity [Member] \| Commercial Revenue [Member]
Segment Information [Abstract]
Revenue			0	0		0	0
Elimination of Intercompany Activity [Member] \| SPINRAZA Royalties [Member]
Segment Information [Abstract]
Revenue			0	0		0	0
Elimination of Intercompany Activity [Member] \| Licensing and Other Royalties [Member]
Segment Information [Abstract]
Revenue			0	0		0	0
Elimination of Intercompany Activity [Member] \| R&D Revenue Under Collaborative Agreements [Member]
Segment Information [Abstract]
Revenue			$ 0	$ (2,930)		$ (5,229)	$ (54,407)