Condensed Consolidated Statements of Operations - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Income Statement [Abstract]
Revenues	$ 2,999,816	$ 2,506,231	$ 5,703,397	$ 5,270,326
Cost of services (including depreciation)	2,552,105	2,150,967	4,882,796	4,582,866
Gross profit	447,711	355,264	820,601	687,460
Equity in earnings of integral unconsolidated affiliates	7,450	1,045	12,633	1,045
Selling, general and administrative expenses	(270,110)	(227,852)	(513,462)	(458,645)
Amortization of intangible assets	(21,291)	(17,779)	(42,646)	(35,687)
Asset impairment charges	(2,319)	0	(2,319)	0
Change in fair value of contingent consideration liabilities	210	2,238	573	(520)
Operating income	161,651	112,916	275,380	193,653
Interest expense	(13,109)	(8,654)	(25,584)	(22,660)
Interest income	2,909	275	3,026	1,034
Other income (expense), net	8,471	3,247	12,143	(6,580)
Income before income taxes	159,922	107,784	264,965	165,447
Provision for income taxes	40,951	32,989	54,675	49,149
Net income	118,971	74,795	210,290	116,298
Less: Net income attributable to non-controlling interests	1,938	849	3,496	3,666
Net income attributable to common stock	$ 117,033	$ 73,946	$ 206,794	$ 112,632
Earnings per share attributable to common stock:
Basic (in dollars per share)	$ 0.83	$ 0.53	$ 1.48	$ 0.79
Diluted (in dollars per share)	$ 0.81	$ 0.52	$ 1.43	$ 0.78
Shares used in computing earnings per share:
Weighted average basic shares outstanding (in shares)	140,276,000	139,856,000	140,199,000	142,154,000
Weighted average diluted shares outstanding (in shares)	144,607,000	143,521,000	144,523,000	145,213,000