Stockholders' Equity, Stock-based Valuation Information (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Employee Stock Options [Member]
Weighted-Average Assumptions [Abstract]
Risk-free interest rate	2.40%	1.80%	1.50%
Dividend yield	0.00%	0.00%	0.00%
Volatility	63.00%	65.90%	58.70%
Expected life	4 years 7 months 6 days	4 years 6 months	4 years 6 months
Board of Director Stock Options [Member]
Weighted-Average Assumptions [Abstract]
Risk-free interest rate	2.80%	2.20%	1.30%
Dividend yield	0.00%	0.00%	0.00%
Volatility	61.50%	61.20%	53.10%
Expected life	6 years 7 months 6 days	6 years 7 months 6 days	6 years 6 months
ESPP [Member]
Weighted-Average Assumptions [Abstract]
Risk-free interest rate	1.80%	0.80%	0.40%
Dividend yield	0.00%	0.00%	0.00%
Volatility	47.30%	59.90%	86.40%
Expected life	6 months	6 months	6 months
Akcea [Member] | Employee Stock Options [Member]
Weighted-Average Assumptions [Abstract]
Risk-free interest rate	2.80%	1.90%	1.60%
Dividend yield	0.00%	0.00%	0.00%
Volatility	77.10%	79.50%	71.40%
Expected life	6 years 29 days	6 years 22 days	6 years 29 days
Akcea [Member] | Board of Director Stock Options [Member]
Weighted-Average Assumptions [Abstract]
Risk-free interest rate	2.90%	1.90%	2.00%
Dividend yield	0.00%	0.00%	0.00%
Volatility	78.20%	79.40%	79.60%
Expected life	6 years 5 months 1 day	6 years 3 months	6 years 29 days