Consolidated Statements of Operations - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Income Statement [Abstract]
Service revenues	$ 6,392,517	$ 7,238,142	$ 6,461,444
Costs of services	3,817,513	4,144,093	3,765,416
Gross margin	2,575,004	3,094,049	2,696,028
Selling, general and administrative expenses	2,107,531	2,117,296	1,951,282
(Income) loss from investments held in employee deferred compensation trusts	(88,020)	86,139	(61,078)
Amortization of intangible assets	2,883	1,667	2,241
Interest income, net	(23,973)	(8,008)	(197)
Income before income taxes	576,583	896,955	803,780
Provision for income taxes	165,437	239,036	205,154
Net income	$ 411,146	$ 657,919	$ 598,626
Net income per share:
Basic (usd per share)	$ 3.90	$ 6.08	$ 5.42
Diluted (usd per share)	$ 3.88	$ 6.03	$ 5.36
Weighted average shares:
Basic (in shares)	105,530	108,214	110,482
Diluted (in shares)	106,074	109,171	111,718
Cash dividends, per share (usd per share)	$ 1.92	$ 1.72	$ 1.52
Dividends declared per share (usd per share)	$ 1.92	$ 1.72	$ 1.52