Income Taxes (Tax Credits) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Netherlands foreign tax credits	$ 9,082	$ 7,695
U.S. general business credits	2,885	2,628
U.S. alternative minimum tax credits	1,321	1,179
Deferred tax credits	13,288	$ 11,502
Future U.S. taxable income needed to realize deferred tax assets	$ 154,600