Income Taxes (Details 2) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Difference between the income tax computation by the company and the statutory body
Provision at the U.S. federal statutory rate	$ 67,163	$ 56,257	$ 46,233
Increase (decrease) resulting from:
State income tax, net of benefit for federal deduction	4,228	3,455	3,492
Foreign income tax rate differential	(538)	(854)	(433)
Employment credits	(421)	(291)	(717)
Changes in valuation allowances	2,713	183	(213)
Nondeductible Goodwill	1,355
Nondeductible Transaction Costs	1,064
Stock-based compensation	282	201	79
Other	2,057	1,575	1,259
Provision for income taxes	$ 77,903	$ 60,526	$ 49,700

Income Tax Reconciliation, Nondeductible Goodwill

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The sum of the current income tax expense or benefit and the deferred income tax expense or benefit pertaining to continuing operations.

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The portion of the difference between total income tax expense or benefit as reported in the Income Statement and the expected income tax expense or benefit computed by applying the domestic federal statutory income tax rates to pretax income from continuing operations attributable to changes in the valuation allowance for deferred tax assets in the period.

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The portion of the difference, between total income tax expense or benefit as reported in the Income Statement for the period and the expected income tax expense or benefit computed by applying the domestic federal statutory income tax rates to pretax income from continuing operations, that is attributable to total foreign income tax expense or benefit. The foreign income tax expense or benefit difference represents the income tax expense or benefit at applicable domestic statutory income tax rates applied to foreign earnings or loss for the period versus the foreign income tax expense or benefit calculated by applying the appropriate foreign tax rates.

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The sum of the differences between total income tax expense or benefit as reported in the Income Statement for the period and the expected income tax expense or benefit computed by applying the domestic federal statutory income tax rates to pretax income from continuing operations attributable to nondeductible expenses under enacted tax laws, or differences in the methodologies used to determine expense amounts for financial statements prepared in accordance with generally accepted accounting principles and enacted tax laws.

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