Significant Accounting Policies, Stock-Based Compensation Expense (Details) $ / shares in Units, $ in Thousands, shares in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019 USD ($) Officer	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) $ / shares	Sep. 30, 2018 USD ($)	Jun. 30, 2019 shares	May 31, 2019 shares
Stock-based Compensation Expense [Abstract]
Number of executive officers terminating employment | Officer	3
Stock-based compensation expense	$ 24,126	$ 34,883	$ 111,564	$ 97,210
Employee Stock Options [Member]
Weighted-Average Assumptions [Abstract]
Risk-free interest rate			2.30%	2.30%
Dividend yield			0.00%	0.00%
Volatility			60.30%	63.10%
Expected life			4 years 7 months 6 days	4 years 7 months 6 days
Unrecognized Compensation Expense [Abstract]
Unrecognized compensation expense related to non-vested stock options	143,900		$ 143,900
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			1.90%	2.80%
Dividend yield			0.00%	0.00%
Volatility			60.70%	61.50%
Expected life			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%
Dividend yield			0.00%	0.00%
Volatility			45.60%	47.30%
Expected life			6 months	6 months
RSUs [Member]
Stock-based Compensation Expense [Abstract]
Vesting period			4 years
Unrecognized Compensation Expense [Abstract]
Unrecognized compensation cost related to non-vested RSUs	71,300		$ 71,300
Weighted average period for recognition			1 year 7 months 6 days
RSUs [Member] | Employees [Member]
Stock-based Compensation Expense [Abstract]
Weighted-average grant date fair value (in dollars per share) | $ / shares			$ 59.79
RSUs [Member] | Board of Directors [Member]
Stock-based Compensation Expense [Abstract]
Weighted-average grant date fair value (in dollars per share) | $ / shares			$ 64.65
2011 Equity Incentive Plan [Member]
Amendment to Equity Plan [Abstract]
Common shares reserved for future issuance (in shares) | shares					23	16
Cost of Products Sold [Member]
Stock-based Compensation Expense [Abstract]
Stock-based compensation expense	127	0	$ 383	$ 0
Research, Development and Patent [Member]
Stock-based Compensation Expense [Abstract]
Stock-based compensation expense	23,744	18,780	71,935	57,698
Selling, General and Administrative [Member]
Stock-based Compensation Expense [Abstract]
Stock-based compensation expense	255	$ 16,103	$ 39,246	$ 39,512
Akcea [Member]
Stock-based Compensation Expense [Abstract]
Stock-based compensation expense	$ (19,100)
Akcea [Member] | Employee Stock Options [Member]
Weighted-Average Assumptions [Abstract]
Risk-free interest rate			2.30%	2.70%
Dividend yield			0.00%	0.00%
Volatility			75.60%	77.10%
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			1.80%	2.90%
Dividend yield			0.00%	0.00%
Volatility			73.80%	78.20%
Expected life			6 years 3 months 18 days	6 years 4 months 24 days
Akcea [Member] | ESPP [Member]
Weighted-Average Assumptions [Abstract]
Risk-free interest rate			2.40%	1.90%
Dividend yield			0.00%	0.00%
Volatility			60.00%	64.20%
Expected life			6 months	6 months