Collaborative Arrangements and Licensing Agreements, GSK (Details) $ in Thousands	1 Months Ended	3 Months Ended
	Mar. 31, 2010 USD ($) PerformanceObligation	Mar. 31, 2018 USD ($) Drug	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)	Jul. 31, 2015 PerformanceObligation
Collaborative Arrangement and Licensing Agreement [Abstract]
Number of separate performance obligations | PerformanceObligation					1
Revenue earned		$ 144,419	$ 115,800
Deferred revenue				$ 233,362
R&D Revenue Under Collaborative Agreements [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Revenue earned		$ 102,396	107,999
GSK [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Upfront payment recorded as deferred revenue	$ 35,000
Number of antisense drugs in development | Drug		2
Maximum amount of payments receivable for license fees and substantive milestones		$ 262,000
Maximum amount of payments receivable for development milestones		47,500
Maximum amount of payments receivable for regulatory milestones		120,000
Maximum amount of payments receivable for commercialization milestones		70,000
Number of separate performance obligations | PerformanceObligation	1
Transaction price	$ 35,000
Deferred revenue		0		$ 0
GSK [Member] | Minimum [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Cumulative payments received		162,000
GSK [Member] | Maximum [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Next prospective milestone		15,000
GSK [Member] | R&D Revenue Under Collaborative Agreements [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Revenue earned		$ 100	$ 6,800