Income Taxes - Reconciliation of Beginning and Ending Amount of Unrecognized Tax Benefits and Related Accrued Interest (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Unrecognized Tax Benefits
Beginning balance	$ 497	$ 535	$ 677
Additions based on tax positions related to the current year	0	0	0
Additions based on tax positions related to the prior years	116	0	0
Decrease based on payments made during the year	0	0	0
Decreases based on tax positions related to the prior years	(420)	(38)	(142)
Ending balance	193	497	535
Accrued Interest and Penalties
Beginning balance	154	202	220
Additions based on tax positions related to the current year	0	0	0
Additions based on tax positions related to the prior years	19	11	24
Decrease based on payments made during the year	0	0	0
Decreases based on tax positions related to the prior years	(145)	(59)	(42)
Ending balance	$ 28	$ 154	$ 202