Income Taxes - Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
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	4,293	$ 2,405	$ 1,360
Additions based on tax positions related to the current year	1,651	1,836	1,045
Additions for tax positions of prior years	1	52
Balance at end of year	5,945	4,293	2,405
Research and development tax credit carryforward
Income Taxes [Line Items]
Unrecognized tax benefits increase	$ 1,700	$ 1,900	$ 1,000