Consolidated Statements of Operations - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Statement [Abstract]
Service revenues	$ 5,109,000	$ 6,074,432	$ 5,800,271
Costs of services	3,096,389	3,549,303	3,389,259
Gross margin	2,012,611	2,525,129	2,411,012
Selling, general and administrative expenses	1,666,041	1,958,295	1,810,601
(Income) loss from investments held in employee deferred compensation trusts (which is completely offset by related costs and expenses - Notes A & I)	(75,188)	(54,917)	11,486
Amortization of intangible assets	1,219	1,361	1,705
Interest income, net	(1,343)	(5,125)	(4,382)
Income before income taxes	421,882	625,515	591,602
Provision for income taxes	115,606	171,082	157,314
Net income	$ 306,276	$ 454,433	$ 434,288
Net income per share:
Basic (usd per share)	$ 2.72	$ 3.93	$ 3.60
Diluted (usd per share)	$ 2.70	$ 3.90	$ 3.57
Shares:
Basic (in shares)	112,729	115,656	120,513
Diluted (in shares)	113,318	116,411	121,602
Dividends declared per share (usd per share)	$ 1.36	$ 1.24	$ 1.12