Organization and Basis of Presentation (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Basis of Presentation and Organization [Abstract]
Cash, Cash Equivalents, and Short-term Investments	$ 389,900		$ 389,900
Accumulated deficit	573,697		573,697		$ 481,270
Net loss	$ 49,577	$ 37,323	92,427	$ 10,136
Net cash used in operating activities			$ (77,528)	$ 49,823
Ionis [Member] | Akcea [Member]
Basis of Presentation and Organization [Abstract]
Ownership percentage by Ionis	76.00%		76.00%