Significant Accounting Policies, Stock-Based Compensation Expense (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Stock-based Compensation Expense [Abstract]
Non-cash stock-based compensation expense	$ 17,586	$ 14,997	$ 56,950	$ 41,907
Stock Options [Member]
Unrecognized Compensation Expense [Abstract]
Unrecognized compensation expense related to non-vested stock options	60,500		$ 60,500
Weighted average period for recognition			1 year 3 months 18 days
RSUs [Member]
Stock-based Compensation Expense [Abstract]
Vesting period			4 years
Unrecognized Compensation Expense [Abstract]
Unrecognized compensation cost related to non-vested RSUs	17,300		$ 17,300
Weighted average period for recognition			1 year 4 months 24 days
RSUs [Member] | Employees [Member]
Stock-based Compensation Expense [Abstract]
Weighted-average grant date fair value (in dollars per share)			$ 42.94
RSUs [Member] | Board of Directors [Member]
Stock-based Compensation Expense [Abstract]
Weighted-average grant date fair value (in dollars per share)			$ 24.42
Research, Development and Patent Expenses [Member]
Stock-based Compensation Expense [Abstract]
Non-cash stock-based compensation expense	13,279	11,297	$ 42,541	32,248
General and Administrative [Member]
Stock-based Compensation Expense [Abstract]
Non-cash stock-based compensation expense	4,307	3,700	14,409	9,659
Akcea Therapeutics [Member]
Stock-based Compensation Expense [Abstract]
Non-cash stock-based compensation expense	$ 2,900	$ 1,300	$ 9,100	$ 2,800