Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income	$ 290,584	$ 343,389	$ 357,796
Adjustments to reconcile net income to net cash provided by operating activities:
Amortization of intangible assets	1,563	1,237	192
Depreciation expense	63,930	63,078	53,273
Stock-based compensation expense—restricted stock and stock units	42,191	42,699	41,292
Excess tax benefits from stock-based compensation	0	(1,822)	(8,762)
Deferred income taxes	44,091	(1,868)	(8,579)
Provision for doubtful accounts	8,022	9,192	12,005
Changes in assets and liabilities, net of effects of acquisitions:
Increase in accounts receivable	(17,039)	(15,888)	(75,745)
Increase in accounts payable, accrued expenses, accrued payroll and benefit costs	47,832	19,726	60,232
(Decrease) increase in income taxes payable, net	(9,655)	(8,246)	19,948
Change in other assets, net of change in other liabilities	(18,528)	(9,416)	(13,416)
Net cash flows provided by operating activities	452,991	442,081	438,236
CASH FLOWS FROM INVESTING ACTIVITIES:
Payments for acquisitions, net of cash acquired	(1,160)	(2,200)	(14,668)
Capital expenditures	(40,753)	(82,956)	(75,057)
Payments to trusts for employee deferred compensation plans	(36,584)	(27,079)	(28,225)
Net cash flows used in investing activities	(78,497)	(112,235)	(117,950)
CASH FLOWS FROM FINANCING ACTIVITIES:
Repurchases of common stock	(231,724)	(176,031)	(271,138)
Cash dividends paid	(121,000)	(114,164)	(107,561)
Decrease in notes payable and other indebtedness	(167)	(154)	(140)
Excess tax benefits from stock-based compensation	0	1,822	8,762
Proceeds from exercises of stock options	0	223	1,529
Net cash flows used in financing activities	(352,891)	(288,304)	(368,548)
Effect of exchange rate changes on cash and cash equivalents	12,949	(5,918)	(14,280)
Net increase (decrease) in cash and cash equivalents	34,552	35,624	(62,542)
Cash and cash equivalents at beginning of period	260,201	224,577	287,119
Cash and cash equivalents at end of period	294,753	260,201	224,577
Cash paid during the year for:
Interest	278	266	285
Income taxes, net of refunds	190,954	219,415	212,668
Non-cash items:
Stock repurchases awaiting settlement	$ 0	$ 14,688	$ 11,935