Condensed Consolidated Statements of Operations (Unaudited) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Income Statement [Abstract]
Net service revenues	$ 1,324,709	$ 1,338,541	$ 3,920,507	$ 3,985,326
Direct costs of services, consisting of payroll, payroll taxes, benefit costs and reimbursable expenses	778,309	786,032	2,309,841	2,343,852
Gross margin	546,400	552,509	1,610,666	1,641,474
Selling, general and administrative expenses	414,179	406,142	1,222,149	1,211,712
Amortization of intangible assets	481	326	1,147	928
Interest income, net	(530)	(283)	(1,108)	(695)
Income before income taxes	132,270	146,324	388,478	429,529
Provision for income taxes	47,570	55,755	144,941	163,928
Net income	$ 84,700	$ 90,569	$ 243,537	$ 265,601
Net income per share:
Basic (in usd per share)	$ 0.69	$ 0.71	$ 1.96	$ 2.07
Diluted (in usd per share)	$ 0.68	$ 0.71	$ 1.94	$ 2.06
Shares:
Basic (in shares)	123,569	127,615	124,567	128,491
Diluted (in shares)	124,200	128,191	125,233	129,216
Cash dividends declared per share (in usd per share)	$ 0.24	$ 0.22	$ 0.72	$ 0.66