Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income	$ 434,288	$ 290,584	$ 343,389
Adjustments to reconcile net income to net cash provided by operating activities:
Amortization of intangible assets	1,705	1,563	1,237
Depreciation expense	64,244	63,930	63,078
Stock-based compensation expense—restricted stock and stock units	44,953	42,191	42,699
Excess tax benefits from stock-based compensation	0	0	(1,822)
Deferred income taxes	(15,885)	44,091	(1,868)
Provision for doubtful accounts	11,914	8,022	9,192
Changes in assets and liabilities, net of effects of acquisitions:
Increase in accounts receivable	(86,217)	(17,039)	(15,888)
Increase in accounts payable, accrued expenses, accrued payroll and benefit costs	89,715	47,832	19,726
Increase (decrease) in income taxes payable, net	28,900	(9,655)	(8,246)
Change in other assets, net of change in other liabilities	(1,295)	(18,528)	(9,416)
Net cash flows provided by operating activities	572,322	452,991	442,081
CASH FLOWS FROM INVESTING ACTIVITIES:
Payments for acquisitions, net of cash acquired	0	(1,160)	(2,200)
Capital expenditures	(42,484)	(40,753)	(82,956)
Payments to trusts for employee deferred compensation plans	(46,025)	(36,584)	(27,079)
Net cash flows used in investing activities	(88,509)	(78,497)	(112,235)
CASH FLOWS FROM FINANCING ACTIVITIES:
Repurchases of common stock	(353,509)	(231,724)	(176,031)
Cash dividends paid	(136,423)	(121,000)	(114,164)
Decrease in notes payable and other indebtedness	(183)	(167)	(154)
Excess tax benefits from stock-based compensation	0	0	1,822
Proceeds from exercises of stock options	0	0	223
Net cash flows used in financing activities	(490,115)	(352,891)	(288,304)
Effect of exchange rate changes on cash and cash equivalents	(11,872)	12,949	(5,918)
Net (decrease) increase in cash and cash equivalents	(18,174)	34,552	35,624
Cash and cash equivalents at beginning of period	294,753	260,201	224,577
Cash and cash equivalents at end of period	276,579	294,753	260,201
Cash paid during the year for:
Interest	233	278	266
Income taxes, net of refunds	137,147	190,954	219,415
Non-cash items:
Stock repurchases awaiting settlement	$ 11,359	$ 0	$ 14,688