Income Taxes - Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Taxes [Line Items]
Interest and penalties accrued	$ 0
Reconciliation of the beginning and ending amount of gross unrecognized tax benefits
Balance at beginning of year	8,619	$ 7,362	$ 5,945
Additions based on tax positions related to the current year	2,889	1,426	1,416
(Reductions) for tax positions of prior years	(8)	(169)
Additions for tax positions of prior years			1
Balance at end of year	11,500	8,619	7,362
Research and development tax credit carryforward
Income Taxes [Line Items]
Unrecognized tax benefits increase	$ 2,900	$ 1,300	$ 1,400