Revenue Arrangements - Sanofi Collaboration Agreement (Narrative) (Details) - Sanofi - USD ($) $ in Millions		1 Months Ended				12 Months Ended			50 Months Ended
	Nov. 27, 2012	Jul. 31, 2016	May 31, 2015	Aug. 31, 2013	Nov. 30, 2012	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2016
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Upfront payment for first indication					$ 2.0
Additional payment for initial indication				$ 3.0
Additional payments received upon exercise of option			$ 2.0
Additional payment upon attaining first milestone for celiac disease indication		$ 1.0
Aggregate payments under Sanofi Agreement									$ 8.0
Initial period to complete research obligations	3 years
Revenue						$ 2.8	$ 2.1	$ 2.1