Collaborative Arrangements and Licensing Agreements, In-Licensing Agreements (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Collaborative Arrangement and Licensing Agreement [Abstract]
Accrued liability	$ 66,618	$ 36,013
University of Massachusetts [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Payments for license fees, milestone payments received and royalty on sales of SPINRAZA	9,700	400
Accrued liability	12,900
Cold Spring Harbor Laboratory [Member]
Collaborative Arrangement and Licensing Agreement [Abstract]
Payments for license fees, milestone payments received and royalty on sales of SPINRAZA	$ 13,100	$ 3,400