Collaboration Agreements - Revenue Recognized from Collaborations (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Collaboration Revenue [Line Items]
Collaboration revenue	$ 4,293	$ 13,192	$ 12,347	$ 23,489
UniQure [Member] | Milestone Payment [Member] | Collaborative Arrangement [Member]
Collaboration Revenue [Line Items]
Collaboration revenue		14		14
Teva [Member] | Recognition of Upfront Payment [Member] | Collaborative Arrangement [Member]
Collaboration Revenue [Line Items]
Collaboration revenue	2,940	3,132	8,724	9,252
Teva [Member] | Research Funding [Member] | Collaborative Arrangement [Member]
Collaboration Revenue [Line Items]
Collaboration revenue	11	84	101	251
Genentech [Member] | Recognition of Upfront Payment [Member] | Collaborative Arrangement [Member]
Collaboration Revenue [Line Items]
Collaboration revenue	182	981	542	2,736
Genentech [Member] | Research Funding [Member] | Collaborative Arrangement [Member]
Collaboration Revenue [Line Items]
Collaboration revenue	910	994	2,730	3,249
Genentech [Member] | Milestone Payment [Member] | Collaborative Arrangement [Member]
Collaboration Revenue [Line Items]
Collaboration revenue	$ 250	$ 7,987	$ 250	$ 7,987