Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income	$ 96,167	$ 78,521
Adjustments to reconcile net income to net cash provided by operating activities:
Amortization of intangible assets	463	301
Depreciation expense	16,253	15,899
Stock-based compensation expense—restricted stock and stock units	10,556	9,857
Deferred income taxes	1,695	10,556
Provision for doubtful accounts	1,778	1,697
Changes in assets and liabilities:
(Increase) decrease in accounts receivable	(49,794)	4,296
Increase (decrease) in accounts payable, accrued expenses, accrued payroll and benefit costs	13,527	(7,498)
Increase in income taxes payable	27,068	27,860
Change in other assets, net of change in other liabilities	(1,495)	(17,989)
Net cash flows provided by operating activities	116,218	123,500
CASH FLOWS FROM INVESTING ACTIVITIES:
Capital expenditures	(7,965)	(10,335)
Payments to trusts for employee deferred compensation plans	(10,585)	(4,939)
Net cash flows used in investing activities	(18,550)	(15,274)
CASH FLOWS FROM FINANCING ACTIVITIES:
Repurchases of common stock	(68,489)	(80,687)
Cash dividends paid	(34,919)	(30,597)
Payments for notes payable and other indebtedness	(44)	(41)
Net cash flows used in financing activities	(103,452)	(111,325)
Effect of exchange rate changes on cash and cash equivalents	3,018	3,041
Net decrease in cash and cash equivalents	(2,766)	(58)
Cash and cash equivalents at beginning of period	294,753	260,201
Cash and cash equivalents at end of period	291,987	260,143
Non-cash items:
Stock repurchases awaiting settlement	$ 0	$ 2,298