Income Taxes (Details 2) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
deferred tax effect - tax cut job act benefit	$ (73,028)	$ 0	$ 0
Difference between the income tax computation by the company and the statutory body
Provision at the U.S. federal statutory rate	76,651	79,580	63,760
Increase (decrease) resulting from:
State income tax, net of benefit for federal deduction	4,472	4,230	4,448
Foreign income tax rate differential	(2,443)	(2,799)	(2,002)
Employment credits	(862)	(821)	(407)
Changes in valuation allowances	629	749	14,667
Nondeductible Goodwill	0	34	4,651
Stock-based compensation	(136)	368	386
Other	278	(1,035)	2,669
Provision for income taxes	$ 5,561	$ 80,306	$ 88,172