Condensed Consolidated Statements of Operations (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Income Statement [Abstract]
Service revenues	$ 1,506,691	$ 1,468,530
Costs of services	895,974	860,942
Gross margin	610,717	607,588
Selling, general and administrative expenses	479,573	461,359
Amortization of intangible assets	338	342
Interest income, net	(957)	(1,496)
Income before income taxes	131,763	147,383
Provision for income taxes	41,848	37,585
Net income	$ 89,915	$ 109,798
Earnings Per Share [Abstract]
Basic (in usd per share)	$ 0.79	$ 0.94
Diluted (in usd per share)	$ 0.79	$ 0.93
Shares:
Basic (in shares)	113,187,000	117,068,000
Diluted (in shares)	113,858,000	117,966,000
Dividends declared per share (in usd per share)	$ 0.34	$ 0.31