Stockholders' Equity, Stock-based Valuation Information (Details)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Employee Stock Options [Member]
Weighted-Average Assumptions [Abstract]
Risk-free interest rate	2.30%	2.40%	1.80%
Dividend yield	0.00%	0.00%	0.00%
Volatility	60.30%	63.00%	65.90%
Expected life	4 years 9 months 18 days	4 years 7 months 6 days	4 years 6 months
Board of Director Stock Options [Member]
Weighted-Average Assumptions [Abstract]
Risk-free interest rate	1.90%	2.80%	2.20%
Dividend yield	0.00%	0.00%	0.00%
Volatility	60.70%	61.50%	61.20%
Expected life	6 years 7 months 6 days	6 years 7 months 6 days	6 years 7 months 6 days
ESPP [Member]
Weighted-Average Assumptions [Abstract]
Risk-free interest rate	2.40%	1.80%	0.80%
Dividend yield	0.00%	0.00%	0.00%
Volatility	45.60%	47.30%	59.90%
Expected life	6 months	6 months	6 months
Akcea [Member] | Employee Stock Options [Member]
Weighted-Average Assumptions [Abstract]
Risk-free interest rate	2.20%	2.80%	1.90%
Dividend yield	0.00%	0.00%	0.00%
Volatility	75.40%	77.10%	79.50%
Expected life	6 years 1 month 2 days	6 years 29 days	6 years 21 days
Akcea [Member] | Board of Director Stock Options [Member]
Weighted-Average Assumptions [Abstract]
Risk-free interest rate	1.80%	2.90%	1.90%
Dividend yield	0.00%	0.00%	0.00%
Volatility	73.80%	78.20%	79.40%
Expected life	6 years 3 months	6 years 5 months 1 day	6 years 3 months
Akcea [Member] | ESPP [Member]
Weighted-Average Assumptions [Abstract]
Risk-free interest rate	2.40%	1.90%	1.10%
Dividend yield	0.00%	0.00%	0.00%
Volatility	60.00%	64.20%	73.30%
Expected life	6 months	6 months	6 months