Income Taxes - Net Deferred Income Tax Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Net deferred income tax liabilities
Deferred income tax assets	$ 56,013	$ 65,690
Deferred income tax liabilities	(25,184)	(33,748)
Valuation allowance	(21,356)	(17,823)
Net deferred income tax assets	$ 9,473	$ 14,119