Collaboration Agreements - Revenue Recognized from Collaborations (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Deferred Revenue Arrangement [Line Items]
Collaboration revenue	$ 28,366	$ 27,352	$ 14,300
UniQure [Member] | Milestone Payment [Member] | Collaborative Arrangement [Member]
Deferred Revenue Arrangement [Line Items]
Collaboration revenue	14	531	198
Teva [Member]
Deferred Revenue Arrangement [Line Items]
Collaboration revenue	0
Teva [Member] | Collaborative Arrangement [Member]
Deferred Revenue Arrangement [Line Items]
Collaboration revenue	12,588	13,773
Teva [Member] | Recognition of Upfront Payment [Member] | Collaborative Arrangement [Member]
Deferred Revenue Arrangement [Line Items]
Collaboration revenue	12,255	13,143	927
Teva [Member] | Research Funding [Member] | Collaborative Arrangement [Member]
Deferred Revenue Arrangement [Line Items]
Collaboration revenue	333	630
Genentech [Member]
Deferred Revenue Arrangement [Line Items]
Collaboration revenue	8,000	5,000	0
Genentech [Member] | Collaborative Arrangement [Member]
Deferred Revenue Arrangement [Line Items]
Collaboration revenue	15,764	12,876	6,948
Genentech [Member] | Milestone Payment [Member] | Collaborative Arrangement [Member]
Deferred Revenue Arrangement [Line Items]
Collaboration revenue	7,913	5,062
Genentech [Member] | Recognition of Upfront Payment [Member] | Collaborative Arrangement [Member]
Deferred Revenue Arrangement [Line Items]
Collaboration revenue	3,603	3,300	3,431
Genentech [Member] | Research Funding [Member] | Collaborative Arrangement [Member]
Deferred Revenue Arrangement [Line Items]
Collaboration revenue	4,248	4,514	3,517
Merck [Member] | Collaborative Arrangement [Member]
Deferred Revenue Arrangement [Line Items]
Collaboration revenue			5,562
Merck [Member] | Research Funding [Member] | Collaborative Arrangement [Member]
Deferred Revenue Arrangement [Line Items]
Collaboration revenue			2,442
Merck [Member] | Recognition of initial payment [Member] | Collaborative Arrangement [Member]
Deferred Revenue Arrangement [Line Items]
Collaboration revenue			1,060
Merck [Member] | Option fee [Member] | Collaborative Arrangement [Member]
Deferred Revenue Arrangement [Line Items]
Collaboration revenue			2,060
Genome BC [Member] | Research Funding [Member] | Collaborative Arrangement [Member]
Deferred Revenue Arrangement [Line Items]
Collaboration revenue		$ 172	$ 665