Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income	$ 657,919	$ 598,626	$ 306,276
Adjustments to reconcile net income to net cash provided by operating activities:
Allowance for credit losses	8,771	9,464	4,200
Depreciation	47,398	52,210	62,281
Amortization of cloud computing implementation costs	28,925	28,023	18,399
Amortization of intangible assets	1,667	2,241	1,219
Realized and unrealized (gains) losses from investments held in employee deferred compensation trusts	98,776	(37,359)	(66,866)
Stock-based compensation	57,663	55,932	52,486
Deferred income taxes	10,810	(21,133)	(13,146)
Changes in operating assets and liabilities, net of effects of acquisitions:
Accounts receivable	(65,626)	(292,628)	127,740
Capitalized cloud computing implementation costs	(40,357)	(31,240)	(33,178)
Accounts payable and accrued expenses	3,735	52,610	1,098
Accrued payroll and benefit cost	(58,067)	99,005	119,231
Employee deferred compensation plan obligations	(61,165)	100,058	13,923
Income taxes payable	2,596	3,587	182
Other assets and liabilities, net	(9,295)	(16,260)	2,683
Net cash flows provided by operating activities	683,750	603,136	596,528
CASH FLOWS FROM INVESTING ACTIVITIES:
Capital expenditures	(61,120)	(36,611)	(33,377)
Investments in employee deferred compensation trusts	(67,388)	(85,432)	(64,351)
Proceeds from employee deferred compensation trust redemptions	30,869	34,434	123,025
Payments for acquisitions, net of cash acquired	(18,984)	0	(15,836)
Net cash flows (used in) provided by investing activities	(116,623)	(87,609)	9,461
CASH FLOWS FROM FINANCING ACTIVITIES:
Repayment of notes payable	0	(239)	(218)
Repurchases of common stock	(319,897)	(287,738)	(159,172)
Dividends paid	(189,286)	(170,612)	(155,935)
Net cash flows used in financing activities	(509,183)	(458,589)	(315,325)
Effect of exchange rate fluctuations	(18,319)	(12,363)	13,284
Change in cash and cash equivalents	39,625	44,575	303,948
Cash and cash equivalents at beginning of period	619,001	574,426	270,478
Cash and cash equivalents at end of period	658,626	619,001	574,426
Cash paid during the year for:
Interest	368	548	577
Income taxes, net of refunds	217,927	219,726	128,321
Non-cash items:
Stock repurchases awaiting settlement	3,504	5,593	3,104
Fund exchanges within employee deferred compensation trusts	103,003	116,815	208,055
Contingent consideration related to acquisition	$ 1,300	$ 0	$ 0