Consolidated Statements of Operations - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Income Statement [Abstract]
Service revenues	$ 6,461,444	$ 5,109,000	$ 6,074,432
Costs of services	3,765,416	3,096,389	3,549,303
Gross margin	2,696,028	2,012,611	2,525,129
Selling, general and administrative expenses	1,951,282	1,666,041	1,958,295
Income from investments held in employee deferred compensation trusts	(61,078)	(75,188)	(54,917)
Amortization of intangible assets	2,241	1,219	1,361
Interest income, net	(197)	(1,343)	(5,125)
Income before income taxes	803,780	421,882	625,515
Provision for income taxes	205,154	115,606	171,082
Net income	$ 598,626	$ 306,276	$ 454,433
Net income per share:
Basic (usd per share)	$ 5.42	$ 2.72	$ 3.93
Diluted (usd per share)	$ 5.36	$ 2.70	$ 3.90
Shares:
Basic (in shares)	110,482	112,729	115,656
Diluted (in shares)	111,718	113,318	116,411
Dividends declared per share (usd per share)	$ 1.52	$ 1.36	$ 1.24