Condensed Consolidated Statements of Operations (Unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Income Statement [Abstract]
Contract revenue		$ 111	$ 6	$ 382
Type of Revenue [Extensible List]	cbio:ContractMember	cbio:ContractMember	cbio:ContractMember	cbio:ContractMember
Operating expenses:
Research and development	$ 3,889	$ 3,401	$ 7,660	$ 5,481
General and administrative	3,225	2,654	6,139	5,017
Total operating expenses	7,114	6,055	13,799	10,498
Loss from operations	(7,114)	(5,944)	(13,793)	(10,116)
Interest and other income, net	632	67	2,269	101
Net loss	(6,482)	(5,877)	(11,524)	(10,015)
Deemed dividend for convertible preferred stock beneficial conversion feature		(3,951)		(3,951)
Net loss attributable to common stockholders	$ (6,482)	$ (9,828)	$ (11,524)	$ (13,966)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.54)	$ (2.53)	$ (1.10)	$ (5.82)
Shares used to compute net loss per share attributable to common stockholders, basic and diluted	11,938,401	3,877,736	10,472,180	2,400,101