Other Income (Expenses) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other Income and Expenses [Abstract]
Interest and investment income	$ 548	$ 72	$ 90
Income tax expense	(214)	(566)	(295)
Other non-operating income	0	126	108
Total other income (expense)	$ 334	$ (368)	$ (97)