Condensed Consolidated Statements of Operations (Unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Income Statement [Abstract]
Service revenues	$ 1,108,326	$ 1,516,385	$ 2,615,017	$ 2,984,915
Costs of services	685,249	878,844	1,581,223	1,739,786
Gross margin	423,077	637,541	1,033,794	1,245,129
Selling, general and administrative expenses	364,828	478,139	844,401	939,498
Amortization of intangible assets	330	341	668	683
Interest income, net	(105)	(1,042)	(1,062)	(2,538)
Income before income taxes	58,024	160,103	189,787	307,486
Provision for income taxes	11,828	45,491	53,676	83,076
Net income	$ 46,196	$ 114,612	$ 136,111	$ 224,410
Net income per share:
Basic (in usd per share)	$ 0.41	$ 0.98	$ 1.20	$ 1.92
Diluted (in usd per share)	$ 0.41	$ 0.98	$ 1.20	$ 1.91
Shares:
Basic (in shares)	112,865,000	116,381,000	113,026,000	116,722,000
Diluted (in shares)	113,121,000	116,988,000	113,489,000	117,475,000
Dividends declared per share (in usd per share)	$ 0.34	$ 0.31	$ 0.68	$ 0.62