Consolidated Statements of Operations (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Statement [Abstract]
Net service revenues	$ 4,695,014	$ 4,245,895	$ 4,111,213
Direct costs of services, consisting of payroll, payroll taxes, benefit costs and reimbursable expenses	2,772,098	2,522,803	2,462,153
Gross margin	1,922,916	1,723,092	1,649,060
Selling, general and administrative expenses	1,425,734	1,324,815	1,305,614
Amortization of intangible assets	557	1,700	398
Interest income, net	(724)	(1,002)	(1,197)
Income before income taxes	497,349	397,579	344,245
Provision for income taxes	191,421	145,384	134,303
Net income	305,928	252,195	209,942
Net income available to common stockholders—diluted	$ 305,928	$ 252,192	$ 208,867
Net income per share:
Basic	$ 2.28	$ 1.85	$ 1.51
Diluted	$ 2.26	$ 1.83	$ 1.50
Shares:
Basic	134,358	136,153	138,201
Diluted	135,541	137,589	139,409
Cash dividends declared per share	$ 0.72	$ 0.64	$ 0.60