Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating expenses:
Research and development	$ 81,206	$ 56,514	$ 41,618
General and administrative	40,326	24,728	16,830
Total operating expenses	121,532	81,242	58,448
Loss from operations	(121,532)	(81,242)	(58,448)
Interest and other income, net	2,027	2,203	1,218
Loss before provision (benefit) for income taxes	(119,505)	(79,039)	(57,230)
Provision (benefit) for income taxes	(14)	10	(9)
Net loss	(119,491)	(79,049)	(57,221)
Other comprehensive gain (loss):
Unrealized gain (loss) on available-for-sale securities	32	335	(418)
Comprehensive loss	$ (119,459)	$ (78,714)	$ (57,639)
Net loss per common share:
Basic and diluted net loss per common share	$ (4.00)	$ (2.75)	$ (2.24)
Weighted-average common shares outstanding used to calculate basic and diluted net loss per common share	29,863	28,732	25,583