Stockholders' Equity, Stock-based Valuation Information (Details) - $ / shares	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Employee Stock Options [Member]
Weighted-Average Assumptions [Abstract]
Risk-free interest rate	1.50%	2.30%	2.40%
Dividend yield	0.00%	0.00%	0.00%
Volatility	58.60%	60.30%	63.00%
Expected life	4 years 8 months 12 days	4 years 9 months 18 days	4 years 7 months 6 days
Board of Director Stock Options [Member]
Weighted-Average Assumptions [Abstract]
Risk-free interest rate	0.50%	1.90%	2.80%
Dividend yield	0.00%	0.00%	0.00%
Volatility	57.60%	60.70%	61.50%
Expected life	6 years 8 months 12 days	6 years 7 months 6 days	6 years 7 months 6 days
ESPP [Member]
Weighted-Average Assumptions [Abstract]
Risk-free interest rate	0.80%	2.40%	1.80%
Dividend yield	0.00%	0.00%	0.00%
Volatility	47.90%	45.60%	47.30%
Expected life	6 months	6 months	6 months
RSUs [Member]
Akcea RSU's [Abstract]
Weighted-average grant date fair value (in dollars per share)	$ 60.86	$ 60.23	$ 51.06
Akcea [Member] \| Employee Stock Options [Member]
Weighted-Average Assumptions [Abstract]
Risk-free interest rate	1.10%	2.20%	2.80%
Dividend yield	0.00%	0.00%	0.00%
Volatility	75.00%	75.40%	77.10%
Expected life	6 years 29 days	6 years 1 month 2 days	6 years 29 days
Akcea [Member] \| Board of Director Stock Options [Member]
Weighted-Average Assumptions [Abstract]
Risk-free interest rate	0.80%	1.80%	2.90%
Dividend yield	0.00%	0.00%	0.00%
Volatility	75.30%	73.80%	78.20%
Expected life	5 years 8 months 1 day	6 years 3 months	6 years 5 months 1 day
Akcea [Member] \| ESPP [Member]
Weighted-Average Assumptions [Abstract]
Risk-free interest rate	1.00%	2.40%	1.90%
Dividend yield	0.00%	0.00%	0.00%
Volatility	71.90%	60.00%	64.20%
Expected life	6 months	6 months	6 months
Akcea [Member] \| RSUs [Member]
Akcea RSU's [Abstract]
Weighted-average grant date fair value (in dollars per share)	$ 15.57	$ 21.95
Akcea [Member] \| RSUs [Member] \| Minimum [Member]
Akcea RSU's [Abstract]
Award term	6 months
Akcea [Member] \| RSUs [Member] \| Maximum [Member]
Akcea RSU's [Abstract]
Award term	4 years

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The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term.

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The risk-free interest rate assumption that is used in valuing an option on its own shares.

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Rate of weighted-average expected volatility for award under share-based payment arrangement.

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Period from grant date that an equity-based award expires, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Expected term of award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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