Income Taxes - Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Interest and penalties accrued	$ 0
Reconciliation of the beginning and ending amount of gross unrecognized tax benefits
Balance at beginning of year	2,405	$ 1,360
Additions based on tax positions related to the current year	1,836	1,045
Additions for tax positions of prior years	52
Balance at end of year	4,293	2,405
Research and development tax credit carryforward
Unrecognized tax benefits increased	$ 1,900	$ 1,000