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Income Taxes - Reconciliation of Income Tax Expense due to the U.S. Federal Statutory Corporate Tax Rate (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Provision at the U.S. federal statutory rate	$ 43,135	$ 76,651	$ 79,580
Increase (decrease) resulting from:
State income tax, net of benefit for federal deduction	3,639	4,472	4,230
Foreign income tax rate differential	(259)	(2,443)	(2,799)
Employment credits	(1,327)	(862)	(821)
Changes in valuation allowances	3,442	629	749
Non-deductible goodwill	0	0	34
Tax Cuts and Jobs Act - Enactment date effect	(575)	(73,028)	0
Stock-based compensation	(74)	(136)	368
Other	(350)	278	(1,035)
Provision for income taxes	$ 47,631	$ 5,561	$ 80,306

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