Income Taxes - Components of Net Deferred Tax Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Deferred tax assets:
Net operating loss carryforwards	$ 25,625	$ 9,254
Research and development tax credits	10,890
Step-up in assets upon conversion to C-corporation	953	1,046
Stock-based compensation expense for non-qualified stock options and restricted stock units	1,401	855
Unrealized losses on marketable securities		284
Deferred rent	258	97
Accruals and other	1,132	568
Total deferred tax assets before valuation allowance	40,259	12,104
Valuation allowance	(40,084)	(12,093)
Total deferred tax assets	175	11
Deferred tax liabilities:
Depreciation	(19)	(11)
Unrealized gains on marketable securities	(156)
Total deferred tax liabilities	(175)	(11)
Net deferred tax assets	$ 0	$ 0