Income Taxes - Net Deferred Income Tax Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Net deferred income tax liabilities
Deferred income tax assets	$ 51,533	$ 56,013
Deferred income tax liabilities	(28,837)	(25,184)
Valuation allowance	(19,814)	(21,356)
Net deferred income tax assets	$ 2,882	$ 9,473