Income Taxes - Summary of Significant Components of Deferred Income Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Net operating loss carryforwards	$ 128,829	$ 34,843
Reserves and accruals	7,840	13,577
Research and development credits	39,344	15,475
Depreciation and amortization		288
Stock-based compensation	7,529	4,098
Total deferred tax assets	183,542	68,281
Deferred tax liabilities:
Depreciation and amortization	(343)
Total deferred tax liabilities	(343)
Valuation allowance	(182,360)	(68,219)
Net deferred tax assets	$ 839	$ 62