Income Taxes - Schedule of Net Deferred Tax Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Deferred tax assets:
Net operating loss carryforwards	$ 50,723	$ 32,997
Research and development tax credit carryforwards	32,522	25,405
Capitalized organization costs	275	306
Stock-based compensation expense	9,178	7,714
Charitable Contributions	11	8
Deferred Revenue	37,499	26,480
Accrued expenses	4,279	4,613
Capitalized research and development expenses	66	73
Total deferred tax assets	134,553	97,596
Deferred tax liabilities:
Depreciation and amortization	(2,544)	(3,470)
Total deferred tax liabilities	(2,544)	(3,470)
Valuation allowance	$ (132,009)	$ (94,126)	$ (82,994)	$ (32,777)