Condensed Consolidated Statements of Operations (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Income Statement [Abstract]
Service revenues	$ 1,712,566	$ 1,189,897	$ 4,691,527	$ 3,804,914
Costs of services	987,239	722,551	2,739,618	2,306,630
Gross margin	725,327	467,346	1,951,909	1,498,284
Selling, general and administrative expenses	495,576	390,799	1,406,731	1,240,879
(Income) loss from investments held in employee deferred compensation trusts (which is completely offset by related costs and expenses - Notes A & I)	1,759	(26,095)	(38,039)	(34,630)
Amortization of intangible assets	572	334	1,724	1,002
Interest income, net	(238)	(202)	(145)	(1,264)
Income before income taxes	227,658	102,510	581,638	292,297
Provision for income taxes	56,787	26,761	150,956	80,437
Net income	$ 170,871	$ 75,749	$ 430,682	$ 211,860
Net income per share:
Basic (in usd per share)	$ 1.55	$ 0.67	$ 3.89	$ 1.88
Diluted (in usd per share)	$ 1.53	$ 0.67	$ 3.85	$ 1.87
Shares:
Basic (in shares)	110,176	112,809	110,816	112,953
Diluted (in shares)	111,490	113,355	111,954	113,444
Dividends declared per share (in usd per share)	$ 0.38	$ 0.34	$ 1.14	$ 1.02