Income Taxes - Reconciliation of Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance as of the beginning of the year	$ 1,594	$ 1,039	$ 656
Increases related to prior year tax positions	101	348	0
Decreases related to prior year tax positions	0	0	(15)
Increases related to current year tax positions	422	283	398
Settlements	0	0	0
Lapse of statute	(327)	(76)	0
Balance as of the end of the year	$ 1,790	$ 1,594	$ 1,039