Condensed Consolidated Statements of Operations (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Income Statement [Abstract]
Service revenues	$ 1,189,897	$ 1,552,132	$ 3,804,914	$ 4,537,047
Costs of services	722,551	905,854	2,306,630	2,648,779
Gross margin	467,346	646,278	1,498,284	1,888,268
Selling, general and administrative expenses	390,799	484,837	1,240,879	1,454,374
Income from investments held in employee deferred compensation trusts (which is completely offset by related costs and expenses -Notes A and H)	(26,095)	(1,450)	(34,630)	(34,628)
Amortization of intangible assets	334	339	1,002	1,022
Interest income, net	(202)	(1,230)	(1,264)	(3,768)
Income before income taxes	102,510	163,782	292,297	471,268
Provision for income taxes	26,761	46,601	80,437	129,677
Net income	$ 75,749	$ 117,181	$ 211,860	$ 341,591
Net income per share:
Basic (in usd per share)	$ 0.67	$ 1.02	$ 1.88	$ 2.94
Diluted (in usd per share)	$ 0.67	$ 1.01	$ 1.87	$ 2.92
Shares:
Basic (in shares)	112,809	115,181	112,953	116,203
Diluted (in shares)	113,355	115,868	113,444	116,934
Dividends declared per share (in usd per share)	$ 0.34	$ 0.31	$ 1.02	$ 0.93