Collaborative Arrangements and Licensing Agreements, Novartis (Details) $ in Thousands, shares in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($) AccountingUnit shares	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Jan. 31, 2017 USD ($)
Collaborative Arrangement and Licensing Agreement [Abstract]
Revenue earned	$ 172,299	$ 120,911	$ 104,152	$ 110,304	$ 160,349	$ 110,927	$ 38,470	$ 36,874	$ 507,666	$ 346,620	$ 283,703
Novartis [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Shares issued (in shares) | shares				1.6
Proceeds from sale of common stock, including premium paid				$ 100,000
Additional amount of common stock required to be purchased				$ 50,000								$ 50,000
Number of units of accounting | AccountingUnit				4
Total consideration under collaboration				$ 180,000
Premium received on shares issued				28,400
Potential premium received if common stock is purchased in the future at a premium				5,000
Consideration allocated to equity				71,600
Consideration allocated to development services and delivery of API for AKCEA-APO(a)-L and AKCEA-APOCIII-L				108,400
Revenue earned									55,200
Deferred revenue	58,900								$ 58,900
Novartis [Member] | Revenue [Member] | Strategic Partner [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Concentration percentage									11.00%
Akcea [Member] | Novartis [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Upfront payment received				75,000
Portion of upfront payment retained				60,000
Portion of upfront payment paid as a sublicense fee				$ 15,000
License fee receivable per drug	150,000								$ 150,000
Next prospective milestone	25,000								$ 25,000
Percentage of license fees, milestone payments and royalties paid as sublicense fee									50.00%
Akcea [Member] | Novartis [Member] | AKCEA-APO(a)-L [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Maximum amount of payments receivable for milestones	600,000								$ 600,000
Maximum amount of payments receivable for development milestones	25,000								25,000
Maximum amount of payments receivable for regulatory milestones	290,000								290,000
Maximum amount of payments receivable for commercialization milestones	285,000								$ 285,000
Royalty percentage received on sales of drug									20.00%
Consideration allocated to development activities	64,000								$ 64,000
Consideration allocated to delivery of API	1,500								1,500
Akcea [Member] | Novartis [Member] | AKCEA-APOCIII-L [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Maximum amount of payments receivable for milestones	530,000								530,000
Maximum amount of payments receivable for development milestones	25,000								25,000
Maximum amount of payments receivable for regulatory milestones	240,000								240,000
Maximum amount of payments receivable for commercialization milestones	265,000								$ 265,000
Royalty percentage received on sales of drug									20.00%
Consideration allocated to development activities	40,100								$ 40,100
Consideration allocated to delivery of API	$ 2,800								$ 2,800