Significant Accounting Policies, Stock-Based Compensation Expense (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Stock-based Compensation Expense [Abstract]
Stock-based compensation expense	$ 28,451	$ 20,912
Research, Development and Patent [Member]
Stock-based Compensation Expense [Abstract]
Stock-based compensation expense	19,682	16,122
Selling, General and Administrative [Member]
Stock-based Compensation Expense [Abstract]
Stock-based compensation expense	$ 8,769	$ 4,790
Employee Stock Options [Member]
Weighted-Average Assumptions [Abstract]
Risk-free interest rate	2.20%	1.80%
Dividend yield	0.00%	0.00%
Volatility	63.20%	66.30%
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	$ 90,700
Weighted average period for recognition	1 year 6 months
Employee Stock Options [Member] | Board of Directors [Member]
Stock-based Compensation Expense [Abstract]
Options granted (in shares)	0	0
Board of Director Stock Options [Member]
Weighted-Average Assumptions [Abstract]
Risk-free interest rate	2.20%	1.30%
Dividend yield	0.00%	0.00%
Volatility	61.20%	53.10%
Expected life	6 years 7 months 6 days	6 years 6 months
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	$ 40,400
Weighted average period for recognition	1 year 10 months 24 days
RSUs [Member] | Employees [Member]
Stock-based Compensation Expense [Abstract]
Weighted-average grant date fair value (in dollars per share)	$ 53.22
RSUs [Member] | Board of Directors [Member]
Stock-based Compensation Expense [Abstract]
Units granted (in shares)	0	0
Akcea Therapeutics [Member]
Stock-based Compensation Expense [Abstract]
Stock-based compensation expense	$ 6,400	$ 3,200