Income Taxes - Components of Income Tax Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Principal components on income tax expense:
Current taxes	$ 1,989	$ 4,482	$ 3,475
Deferred taxes	9,405	(23)	1,378
Income tax expense (benefit)	$ 11,394	$ 4,459	$ 4,853