Income Taxes - Net Deferred Tax Assets (Detail) - USD ($)	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Net operating loss carryforwards	$ 48,496	$ 36,880
Research and development tax credit carryforwards	7,725	7,078
Accruals and other temporary differences	3,578	4,897
Debt discount	(14,630)	0
Capitalized research and development expenses, net	29,673	34,579
Total deferred tax assets	74,842	83,434
Valuation allowance	(74,842)	(83,434)	$ (54,773)	$ (33,826)
Net deferred tax asset	$ 0	$ 0