Strategic Alliances and Collaborations - Revenue from Strategic Alliances (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenue from External Customer [Line Items]
Total revenues	$ 18	$ 18	$ 18	$ 18	$ 18	$ 204	$ 483	$ 489	$ 72	$ 1,194	$ 20,759
Strategic Alliances and Collaborations
Revenue from External Customer [Line Items]
Total revenues									72	1,194	20,759
Strategic Alliances and Collaborations | Sanofi
Revenue from External Customer [Line Items]
Total revenues									72	72	71
Strategic Alliances and Collaborations | AstraZeneca
Revenue from External Customer [Line Items]
Total revenues									0	1,122	18,871
Strategic Alliances and Collaborations | Biogen
Revenue from External Customer [Line Items]
Total revenues									$ 0	$ 0	$ 1,817