Collaboration Agreements - Revenue Recognized from Collaborations (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Deferred Revenue Arrangement [Line Items]
Collaboration revenue	$ 4,044	$ 5,296	$ 8,054	$ 10,297
Teva [Member] | Recognition of Upfront Payment [Member] | Collaborative Arrangement [Member]
Deferred Revenue Arrangement [Line Items]
Collaboration revenue	2,908	3,095	5,784	6,120
Teva [Member] | Research Funding [Member] | Collaborative Arrangement [Member]
Deferred Revenue Arrangement [Line Items]
Collaboration revenue	45	87	90	167
Genentech [Member] | Recognition of Upfront Payment [Member] | Collaborative Arrangement [Member]
Deferred Revenue Arrangement [Line Items]
Collaboration revenue	181	969	360	1,755
Genentech [Member] | Research Funding [Member] | Collaborative Arrangement [Member]
Deferred Revenue Arrangement [Line Items]
Collaboration revenue	$ 910	$ 1,145	$ 1,820	$ 2,255