Income Taxes - Summary of Significant Components of Deferred Income Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Deferred tax assets:
Net operating loss carryforwards	$ 263,512	$ 128,829
Reserves and accruals	10,425	7,840
Research and development credits	73,442	39,344
Lease Obligation	81,639
Stock-based compensation	17,494	7,529
Total deferred tax assets	446,512	183,542
Deferred tax liabilities:
Right-of-use asset	(72,243)
Depreciation and amortization	(10,916)	(343)
Total deferred tax liabilities	(83,159)	(343)
Valuation allowance	(361,233)	(182,360)
Net deferred tax assets	$ 2,120	$ 839