Income Taxes - Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Unrecognized Tax Benefits [Roll Forward]
Beginning of year balance	$ 43	$ 613	$ 613
Additions for tax positions of prior years	130	2	0
Additions for tax positions of the current year	12	21	0
Reduction for prior year tax positions	0	(48)	0
Reductions for settlements	0	(545)	0
End of year balance	$ 185	$ 43	$ 613