Income Taxes - Summary of Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized benefit—beginning of year	$ 800	$ 684
Gross increases (decreases)—prior year tax positions	(7)	0
Gross increases (decreases)—current year tax positions	264	116
Unrecognized benefit—end of year	$ 1,057	$ 800