Income Taxes - Changes Occurred in the Amount of Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Reconciliation of unrecognized tax benefits including related interest and penalties
Balance at beginning of year	$ 1,469	$ 634
Additions for current year tax provisions	51	49
Additions/Reduction for tax positions of prior years	0	786
Reduction for prior year tax provisions	0	0
Balance at end of year	$ 1,520	$ 1,469