Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income	$ 306,276	$ 454,433	$ 434,288
Adjustments to reconcile net income to net cash provided by operating activities:
Allowance for credit losses	4,200	9,868	11,914
Depreciation	62,281	64,264	64,244
Amortization of cloud computing implementation costs	18,399	3,624	0
Amortization of intangible assets	1,219	1,361	1,705
Realized and unrealized (gains) losses from investments held in employee deferred compensation trusts	(66,866)	(44,492)	22,343
Stock-based compensation	52,486	48,300	44,953
Deferred income taxes	(13,146)	(9,473)	(15,885)
Changes in operating assets and liabilities, net of effects of acquisitions:
Accounts receivable	127,740	(48,461)	(86,217)
Capitalized cloud computing implementation costs	(33,178)	(30,338)	0
Accounts payable and accrued expenses	1,098	(9,204)	32,428
Accrued payroll and benefit costs	119,231	17,705	13,845
Employee deferred compensation plan obligations	13,923	87,670	21,099
Income taxes payable	182	(18,798)	28,900
Other assets and liabilities, net	2,683	(6,830)	(1,295)
Net cash flows provided by operating activities	596,528	519,629	572,322
CASH FLOWS FROM INVESTING ACTIVITIES:
Capital expenditures	(33,377)	(59,464)	(42,484)
Investments in employee deferred compensation trusts	(64,351)	(71,432)	(69,716)
Proceeds from employee deferred compensation trust redemptions	123,025	28,758	23,691
Payments for acquisitions, net of cash acquired	(15,836)	0	0
Net cash flows provided by (used in) investing activities	9,461	(102,138)	(88,509)
CASH FLOWS FROM FINANCING ACTIVITIES:
Repayment of notes payable	(218)	(200)	(183)
Repurchases of common stock	(159,172)	(277,535)	(353,509)
Dividends paid	(155,935)	(145,631)	(136,423)
Net cash flows used in financing activities	(315,325)	(423,366)	(490,115)
Effect of exchange rate fluctuations	13,284	(226)	(11,872)
Change in cash and cash equivalents	303,948	(6,101)	(18,174)
Cash and cash equivalents at beginning of period	270,478	276,579	294,753
Cash and cash equivalents at end of period	574,426	270,478	276,579
Cash paid during the year for:
Interest	577	232	233
Income taxes, net of refunds	128,321	191,522	137,147
Non-cash items:
Stock repurchases awaiting settlement	3,104	6,469	11,359
Fund exchanges within employee deferred compensation trusts	$ 208,055	$ 41,648	$ 79,300