Significant Accounting Policies, Stock-Based Compensation Expense (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Stock-based Compensation Expense [Abstract]
Stock-based compensation expense	$ 33,876	$ 21,258	$ 62,327	$ 42,170
Research, Development and Patent [Member]
Stock-based Compensation Expense [Abstract]
Stock-based compensation expense	19,236	16,140	38,918	32,262
Selling, General and Administrative [Member]
Stock-based Compensation Expense [Abstract]
Stock-based compensation expense	14,640	5,118	$ 23,409	$ 9,908
Employee Stock Options [Member]
Weighted-Average Assumptions [Abstract]
Risk-free interest rate			2.30%	1.80%
Dividend yield			0.00%	0.00%
Volatility			63.10%	66.10%
Expected life			4 years 7 months 6 days	4 years 6 months
Unrecognized Compensation Expense [Abstract]
Unrecognized compensation expense related to non-vested stock options	151,800		$ 151,800
Weighted average period for recognition			1 year 4 months 24 days
Employee Stock Options [Member] | Board of Directors [Member]
Stock-based Compensation Expense [Abstract]
Options granted (in shares)			0	0
ESPP [Member]
Weighted-Average Assumptions [Abstract]
Risk-free interest rate			1.60%	0.70%
Dividend yield			0.00%	0.00%
Volatility			44.40%	66.50%
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	33,600		$ 33,600
Weighted average period for recognition			1 year 8 months 12 days
RSUs [Member] | Employees [Member]
Stock-based Compensation Expense [Abstract]
Weighted-average grant date fair value (in dollars per share)			$ 52.83
RSUs [Member] | Board of Directors [Member]
Stock-based Compensation Expense [Abstract]
Units granted (in shares)			0	0
Akcea [Member]
Stock-based Compensation Expense [Abstract]
Stock-based compensation expense	$ 12,100	$ 3,900	$ 18,500	$ 7,100