Cash purchase price for each equity award held, including stock options and restricted stock units (RSUs).

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The number of executive officers terminating employment with the company.

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The remaining portion of the available equity pool to be assumed by the acquiring parent company and used to make equity grants to legacy employees.

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Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

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- Definition

Amount of cost not yet recognized for nonvested award under share-based payment arrangement.

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Weighted-average period over which cost not yet recognized is expected to be recognized for 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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Amount of cost to be recognized for nonvested award under share-based payment arrangement. Excludes share and unit options.

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Amount of cost to be recognized for option under share-based payment arrangement.

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Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition.

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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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- Definition

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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- Definition

The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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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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Significant Accounting Policies, Stock-Based Compensation Expense (Details) $ / shares in Units, $ in Thousands, shares in Millions	1 Months Ended	3 Months Ended		9 Months Ended
	Oct. 31, 2020 $ / shares shares	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($) Officer	Sep. 30, 2020 USD ($) $ / shares	Sep. 30, 2019 USD ($)
Weighted-Average Assumptions [Abstract]
Cash purchase price of equity awards (in dollars per share) \| $ / shares	$ 18.15
Stock-based Compensation Expense [Abstract]
Stock-based compensation expense		$ 45,845	$ 24,126	$ 135,077	$ 111,564
Cost of Products Sold [Member]
Stock-based Compensation Expense [Abstract]
Stock-based compensation expense		315	127	902	383
Research, Development and Patent [Member]
Stock-based Compensation Expense [Abstract]
Stock-based compensation expense		25,359	23,744	76,931	71,935
Selling, General and Administrative [Member]
Stock-based Compensation Expense [Abstract]
Stock-based compensation expense		20,171	255	$ 57,244	$ 39,246
Employee Stock Options [Member]
Weighted-Average Assumptions [Abstract]
Risk-free interest rate				1.50%	2.30%
Dividend yield				0.00%	0.00%
Volatility				58.80%	60.30%
Expected life				4 years 8 months 12 days	4 years 7 months 6 days
Unrecognized Compensation Expense [Abstract]
Unrecognized compensation expense related to non-vested stock options		118,200		$ 118,200
Weighted average period for recognition				1 year 2 months 12 days
Board of Director Stock Options [Member]
Weighted-Average Assumptions [Abstract]
Risk-free interest rate				0.50%	1.90%
Dividend yield				0.00%	0.00%
Volatility				57.60%	60.70%
Expected life				6 years 8 months 12 days	6 years 7 months 6 days
ESPP [Member]
Weighted-Average Assumptions [Abstract]
Risk-free interest rate				0.80%	2.40%
Dividend yield				0.00%	0.00%
Volatility				47.90%	45.60%
Expected life				6 months	6 months
RSUs [Member]
Weighted-Average Assumptions [Abstract]
Vesting period				4 years
Unrecognized Compensation Expense [Abstract]
Unrecognized compensation cost related to non-vested RSUs		87,300		$ 87,300
Weighted average period for recognition				1 year 6 months
RSUs Granted to Employees [Member]
Weighted-Average Assumptions [Abstract]
Weighted-average grant date fair value (in dollars per share) \| $ / shares				$ 61.72
RSUs Granted to Board of Directors [Member]
Weighted-Average Assumptions [Abstract]
Weighted-average grant date fair value (in dollars per share) \| $ / shares				$ 60.20
Akcea [Member]
Weighted-Average Assumptions [Abstract]
Remaining portion of available equity share pool (in shares) \| shares	2.8
Unrecognized Compensation Expense [Abstract]
Unrecognized compensation cost related to non-vested options and RSUs		59,700		$ 59,700
Stock-based Compensation Expense [Abstract]
Stock-based compensation expense		$ 13,800	$ (3,500)	$ 37,200	$ 29,500
Akcea [Member] \| Executive Officers Terminating Employment [Member]
Stock-based Compensation Expense [Abstract]
Number of executive officers terminating employment \| Officer			3
Stock-based compensation expense			$ (19,100)
Akcea [Member] \| Employee Stock Options [Member]
Weighted-Average Assumptions [Abstract]
Risk-free interest rate				1.10%	2.30%
Dividend yield				0.00%	0.00%
Volatility				75.00%	75.60%
Expected life				6 years 1 month 6 days	6 years 1 month 6 days
Akcea [Member] \| Board of Director Stock Options [Member]
Weighted-Average Assumptions [Abstract]
Risk-free interest rate				0.80%	1.80%
Dividend yield				0.00%	0.00%
Volatility				75.30%	73.80%
Expected life				5 years 8 months 12 days	6 years 3 months 18 days
Akcea [Member] \| ESPP [Member]
Weighted-Average Assumptions [Abstract]
Risk-free interest rate				1.00%	2.40%
Dividend yield				0.00%	0.00%
Volatility				71.90%	60.00%
Expected life				6 months	6 months
Akcea [Member] \| RSUs [Member]
Weighted-Average Assumptions [Abstract]
Weighted-average grant date fair value (in dollars per share) \| $ / shares				$ 15.57
Akcea [Member] \| RSUs [Member] \| Minimum [Member]
Weighted-Average Assumptions [Abstract]
Vesting period				6 months
Akcea [Member] \| RSUs [Member] \| Maximum [Member]
Weighted-Average Assumptions [Abstract]
Vesting period				4 years