Condensed Consolidated Statements of Operations (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Income Statement [Abstract]
Service revenues	$ 1,580,581	$ 1,108,326	$ 2,978,961	$ 2,615,017
Costs of services	915,709	691,791	1,752,378	1,584,094
Gross margin	664,872	416,535	1,226,583	1,030,923
Selling, general and administrative expenses	488,093	407,213	911,155	850,081
(Income) loss from investments held in employee deferred compensation trusts (which is completely offset by related costs and expenses - Notes A & I)	(27,810)	(48,927)	(39,797)	(8,551)
Amortization of intangible assets	576	330	1,152	668
Interest expense (income), net	151	(105)	105	(1,062)
Income before income taxes	203,862	58,024	353,968	189,787
Provision for income taxes	54,649	11,828	94,157	53,676
Net income	$ 149,213	$ 46,196	$ 259,811	$ 136,111
Net income per share:
Basic (in usd per share)	$ 1.35	$ 0.41	$ 2.34	$ 1.20
Diluted (in usd per share)	$ 1.33	$ 0.41	$ 2.32	$ 1.20
Shares:
Basic (in shares)	110,861	112,865	111,141	113,026
Diluted (in shares)	111,889	113,121	112,191	113,489
Dividends declared per share (in usd per share)	$ 0.38	$ 0.34	$ 0.76	$ 0.68