Income Taxes - Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
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	7,362	$ 5,945	$ 4,293
Additions based on tax positions related to the current year	1,426	1,416	1,651
Additions (reductions) for tax positions of prior years	(169)
Additions for tax positions of prior years		1	1
Balance at end of year	8,619	7,362	5,945
Research and development tax credit carryforward
Income Taxes [Line Items]
Unrecognized tax benefits increase	$ 1,300	$ 1,400	$ 1,700