Significant Accounting Policies, Stock-Based Compensation Expense (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	May 31, 2017	Apr. 30, 2017
Stock-based Compensation Expense [Abstract]
Stock-based compensation expense	$ 21,258	$ 19,260	$ 42,170	$ 39,364
Research, Development and Patent [Member]
Stock-based Compensation Expense [Abstract]
Stock-based compensation expense	16,140	14,492	32,262	29,262
Selling, General and Administrative [Member]
Stock-based Compensation Expense [Abstract]
Stock-based compensation expense	5,118	4,768	$ 9,908	$ 10,102
Employee Stock Options [Member]
Weighted-Average Assumptions [Abstract]
Risk-free interest rate			1.80%	1.50%
Dividend yield			0.00%	0.00%
Volatility			66.10%	58.40%
Expected life			4 years 6 months	4 years 6 months
Unrecognized Compensation Expense [Abstract]
Unrecognized compensation expense related to non-vested stock options	85,900		$ 85,900
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			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	21,700		$ 21,700
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)			$ 47.85
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	$ 3,900	$ 3,100	$ 7,100	$ 6,300
2011 Equity Incentive Plan [Member]
Amendment to Equity Plan [Abstract]
Common shares reserved for future issuance (in shares)					16,000,000	11,000,000