Consolidated Statements of Operations - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Statement [Abstract]
Service revenues	$ 6,074,432	$ 5,800,271	$ 5,266,789
Costs of services	3,543,913	3,390,257	3,102,977
Gross margin	2,530,519	2,410,014	2,163,812
Selling, general and administrative expenses	1,908,768	1,821,089	1,646,532
Amortization of intangible assets	1,361	1,705	1,563
Interest income, net	(5,125)	(4,382)	(1,799)
Income before income taxes	625,515	591,602	517,516
Provision for income taxes	171,082	157,314	226,932
Net income	$ 454,433	$ 434,288	$ 290,584
Net income per share:
Basic (usd per share)	$ 3.93	$ 3.60	$ 2.34
Diluted (usd per share)	$ 3.90	$ 3.57	$ 2.33
Shares:
Basic (in shares)	115,656	120,513	124,152
Diluted (in shares)	116,411	121,602	124,892
Dividends declared per share (usd per share)	$ 1.24	$ 1.12	$ 0.96