Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income	$ 205,482	$ 158,837
Adjustments to reconcile net income to net cash provided by operating activities:
Amortization of intangible assets	905	666
Depreciation expense	32,403	31,845
Stock-based compensation expense—restricted stock and stock units	21,945	20,727
Deferred income taxes	(10,188)	18
Provision for doubtful accounts	3,950	3,421
Changes in assets and liabilities:
(Increase) decrease in accounts receivable	(80,580)	3,376
Increase in accounts payable, accrued expenses, accrued payroll and benefit costs	45,959	38
Increase in income taxes payable	36,215	49,229
Change in other assets, net of change in other liabilities	7,873	(10,097)
Net cash flows provided by operating activities	263,964	258,060
CASH FLOWS FROM INVESTING ACTIVITIES:
Payments for acquisitions, net of cash acquired	0	(1,643)
Capital expenditures	(17,261)	(21,186)
Payments to trusts for employee deferred compensation plans	(19,827)	(15,245)
Net cash flows used in investing activities	(37,088)	(38,074)
CASH FLOWS FROM FINANCING ACTIVITIES:
Repurchases of common stock	(138,002)	(129,243)
Cash dividends paid	(68,985)	(61,448)
Payments for notes payable and other indebtedness	(90)	(82)
Net cash flows used in financing activities	(207,077)	(190,773)
Effect of exchange rate changes on cash and cash equivalents	(5,896)	8,384
Net increase in cash and cash equivalents	13,903	37,597
Cash and cash equivalents at beginning of period	294,753	260,201
Cash and cash equivalents at end of period	308,656	297,798
Non-cash items:
Stock repurchases awaiting settlement	$ 6,483	$ 1,255