Consolidated and Combined Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating expenses:
Research and development	$ 56,514	$ 41,618	$ 15,446
General and administrative	24,728	16,830	12,710
Total operating expenses	81,242	58,448	28,156
Loss from operations	(81,242)	(58,448)	(28,156)
Interest and other income, net	2,203	1,218	125
Loss before provision (benefit) for income taxes	(79,039)	(57,230)	(28,031)
Provision (benefit) for income taxes	10	(9)	(25)
Net loss	(79,049)	(57,221)	(28,006)
Other comprehensive gain (loss):
Unrealized gain (loss) on available-for-sale securities	335	(418)	(100)
Comprehensive loss	$ (78,714)	$ (57,639)	$ (28,106)
Net loss per common share:
Basic and diluted net loss per common share	$ (2.75)	$ (2.24)	$ (5.62)
Weighted-average common shares outstanding used to calculate basic and diluted net loss per common share	28,732	25,583	4,986