Consolidated Statements of Operations - USD ($) shares in Thousands	3 Months Ended								8 Months Ended	12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Dec. 31, 2016	Dec. 31, 2015
Income Statement [Abstract]
Collaboration revenue	$ 5,627,000	$ 4,869,000	$ 4,206,000	$ 1,777,000	$ 1,693,000	$ 1,688,000	$ 1,377,000	$ 1,286,000		$ 16,479,000	$ 6,044,000
Operating expenses:
Research and development	11,331,000	7,861,000	7,423,000	5,225,000	4,375,000	3,458,000	1,966,000	1,371,000	$ 1,105,000	31,840,000	11,170,000
In-process research and development									6,055,000
General and administrative	5,118,000	4,705,000	3,729,000	3,246,000	2,809,000	1,531,000	2,833,000	1,110,000	2,379,000	16,798,000	8,283,000
Total operating expenses	16,449,000	12,566,000	11,152,000	8,471,000	7,184,000	4,989,000	4,799,000	2,481,000	9,539,000	48,638,000	19,453,000
Operating loss	(10,822,000)	(7,697,000)	(6,946,000)	(6,694,000)	(5,491,000)	(3,301,000)	(3,422,000)	(1,195,000)	(9,539,000)	(32,159,000)	(13,409,000)
Interest income	259,000	215,000	46,000	5,000						525,000
Loss before income taxes	(10,563,000)	(7,482,000)	(6,900,000)	(6,689,000)	(5,491,000)	(3,301,000)	(3,422,000)	(1,195,000)	(9,539,000)	(31,634,000)	(13,409,000)
Benefit from income taxes					246,000	282,000	382,000	102,000		0	1,012,000
Net loss	$ (10,563,000)	$ (7,482,000)	$ (6,900,000)	$ (6,689,000)	$ (5,245,000)	$ (3,019,000)	$ (3,040,000)	$ (1,093,000)	$ (9,539,000)	$ (31,634,000)	$ (12,397,000)
Net loss per share or common unit, basic and diluted	$ (0.31)	$ (0.22)	$ (0.36)	$ (9.89)	$ (9.57)	$ (10.27)	$ (2.37)	$ (0.85)	$ (11.55)	$ (1.42)	$ (51.02)
Weighted average shares or common units outstanding, basic and diluted	34,507	34,316	19,121	676	548	294	1,284	1,284	826	22,222	243