Significant Accounting Policies, Stock-Based Compensation Expense (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Stock-based Compensation Expense [Abstract]
Non-cash stock-based compensation expense	$ 20,912	$ 20,103
Research, Development and Patent [Member]
Stock-based Compensation Expense [Abstract]
Non-cash stock-based compensation expense	16,122	14,770
Selling, General and Administrative [Member]
Stock-based Compensation Expense [Abstract]
Non-cash stock-based compensation expense	4,790	5,333
Akcea Therapeutics [Member]
Stock-based Compensation Expense [Abstract]
Non-cash stock-based compensation expense	$ 3,200	$ 3,200
Employee Stock Options [Member]
Weighted-Average Assumptions [Abstract]
Risk-free interest rate	1.80%	1.50%
Dividend yield	0.00%	0.00%
Volatility	66.30%	57.90%
Expected life	4 years 6 months	4 years 6 months
Unrecognized Compensation Expense [Abstract]
Unrecognized compensation expense related to non-vested stock options	$ 91,800
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
ESPP [Member]
Weighted-Average Assumptions [Abstract]
Risk-free interest rate	0.70%	0.50%
Dividend yield	0.00%	0.00%
Volatility	66.50%	69.40%
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	$ 25,600
Weighted average period for recognition	1 year 9 months 18 days
RSUs [Member] | Employees [Member]
Stock-based Compensation Expense [Abstract]
Weighted-average grant date fair value (in dollars per share)	$ 48.09
RSUs [Member] | Board of Directors [Member]
Stock-based Compensation Expense [Abstract]
Units granted (in shares)	0	0