Consolidated Statements of Operations - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Revenue:
Collaboration revenue—related party	$ 65,201	$ 71,644
Operating expenses:
Research and development	70,052	67,798
General and administrative	26,443	23,061
Total operating expenses	96,495	90,859
Operating loss	(31,294)	(19,215)
Other income, net	3,961	2,808
Loss before provision for income taxes	(27,333)	(16,407)
Provision for income taxes	46	36
Net loss	(27,379)	(16,443)
Reconciliation of net loss to net loss attributable to common stockholders:
Net loss	(27,379)	(16,443)
Net loss attributable to common stockholders	$ (27,379)	$ (17,237)
Net loss per share attributable to common stockholders, basic and diluted (in dollars per share)	$ (0.84)	$ (0.57)
Weighted-average common shares outstanding, basic and diluted (in shares)	32,567	30,055
Series A Convertible Preferred Stock
Reconciliation of net loss to net loss attributable to common stockholders:
Accrued dividends on convertible preferred stock	$ 0	$ (268)
Series B Convertible Preferred Stock
Reconciliation of net loss to net loss attributable to common stockholders:
Accrued dividends on convertible preferred stock	0	(318)
Series B-1 Convertible Preferred Stock
Reconciliation of net loss to net loss attributable to common stockholders:
Accrued dividends on convertible preferred stock	$ 0	$ (208)