Income Taxes - Significant Components of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred tax assets:
Net operating loss carryforwards	$ 784,721	$ 416,709
Research tax credits	269,658	167,489
Reserves, accruals, and other	18,106	15,960
Lease obligation	44,446	52,734
Share-based compensation	96,932	133,067
Total deferred tax assets	1,213,863	785,959
Less: valuation allowance	(1,176,910)	(737,003)
Deferred tax assets, net of valuation allowance	36,953	48,956
Deferred tax liabilities:
Depreciation and amortization	(34,576)	(46,398)
Prepaid expenses	(1,523)	(1,862)
Total deferred tax liabilities	(36,099)	(48,260)
Net deferred tax assets	$ 854	$ 696