Income Taxes - Schedule of Net Deferred Tax Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Deferred tax assets:
Net operating loss carryforwards	$ 18,481	$ 25,293
Research and development tax credit carryforwards	14,991	4,180
Capitalized organization costs	483	527
Stock-based compensation expense	5,624	1,897
Charitable Contributions	6	5
Deferred Revenue	42,742
Accrued expenses	5,560	977
Capitalized research and development expenses	115	126
Total deferred tax assets	88,002	33,005
Deferred tax liabilities:
Depreciation and amortization	(5,008)	(228)
Total deferred tax liabilities	(5,008)	(228)
Valuation allowance	$ (82,994)	(32,777)	$ (10,522)	$ (4,294)
Net deferred tax assets		$ 0