Nature of Business and Overview - (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) Program product	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Nature of Business and Overview
Number of drug candidates in clinical development | product									7
Cash, cash equivalents and marketable securities	$ 139,400								$ 139,400
Net Loss	$ (3,841)	$ (26,363)	$ (28,566)	$ (34,261)	$ (32,307)	$ (29,598)	$ (31,952)	$ (34,673)	(93,031)	$ (128,530)	$ (127,197)
Net cash used in operations									$ 99,931	$ 113,036	$ 98,887
Minimum number of programs at risk due to fund restriction | Program									1