Selected Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ 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
Total revenue	$ 0	$ 0	$ 0	$ 425	$ 825	$ 28,650	$ 277	$ 229	$ 425	$ 29,981	$ 990
Total operating expenses	6,593	9,150	7,350	6,587	4,447	9,120	6,769	6,779	29,680	27,115	36,186
Income (loss) from operations	(6,593)	(9,150)	(7,350)	(6,162)	(3,622)	19,530	(6,492)	(6,550)	(29,255)	2,866	(35,196)
Net (loss) income	$ (6,547)	$ (9,105)	$ (7,316)	$ (6,061)	$ (3,531)	$ 19,487	$ (6,491)	$ (7,574)	$ (29,029)	$ 1,891	$ (33,452)
Net loss per share—basic and diluted (in dollars per share)	$ (0.22)	$ (0.37)	$ (0.30)	$ (0.25)					$ (1.11)
Net income (loss) per share—basic (in dollars per share)					$ (0.15)	$ 0.95	$ (0.33)	$ (0.39)	(1.11)	$ 0.09	$ (1.76)
Net income (loss) per share—diluted (in dollars per share)					$ (0.15)	$ 0.91	$ (0.33)	$ (0.39)	$ (1.11)	$ 0.09	$ (1.76)
Revenues									$ 0	$ 406	$ 490
Transaction costs incurred in relation to business combination										$ 2,500
Viventia Bio Inc.
Transaction costs incurred in relation to business combination						$ 2,500
Roche
Revenues						$ 29,000			$ 400