Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income	$ 320,724	$ 243,537
Adjustments to reconcile net income to net cash provided by operating activities:
Amortization of intangible assets	1,334	1,147
Depreciation expense	48,344	47,916
Stock-based compensation expense—restricted stock and stock units	33,000	31,163
Deferred income taxes	(8,427)	(13,309)
Provision for doubtful accounts	8,210	5,327
Changes in assets and liabilities:
Increase in accounts receivable	(119,096)	(21,810)
Increase in accounts payable, accrued expenses, accrued payroll and benefit costs	131,874	65,745
Increase in income taxes payable	18,339	32,320
Change in other assets, net of change in other liabilities	14,975	(3,573)
Net cash flows provided by operating activities	449,277	388,463
CASH FLOWS FROM INVESTING ACTIVITIES:
Payments for acquisitions, net of cash acquired	0	(1,699)
Capital expenditures	(27,186)	(28,540)
Payments to trusts for employee deferred compensation plans	(27,984)	(23,837)
Net cash flows used in investing activities	(55,170)	(54,076)
CASH FLOWS FROM FINANCING ACTIVITIES:
Repurchases of common stock	(216,401)	(167,342)
Cash dividends paid	(102,696)	(91,078)
Payments for notes payable and other indebtedness	(136)	(124)
Net cash flows used in financing activities	(319,233)	(258,544)
Effect of exchange rate changes on cash and cash equivalents	(7,887)	12,292
Net increase in cash and cash equivalents	66,987	88,135
Cash and cash equivalents at beginning of period	294,753	260,201
Cash and cash equivalents at end of period	361,740	348,336
Non-cash items:
Stock repurchases awaiting settlement	$ 7,043	$ 0