Condensed Consolidated Statements of Operations - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Income Statement [Abstract]
Revenues	$ 2,506,231	$ 2,839,199	$ 5,270,326	$ 5,646,458
Cost of services (including depreciation)	2,150,967	2,519,694	4,582,866	4,962,972
Gross profit	355,264	319,505	687,460	683,486
Equity in earnings of integral unconsolidated affiliates	1,045	0	1,045	0
Selling, general and administrative expenses	(227,852)	(223,944)	(458,645)	(455,852)
Amortization of intangible assets	(17,779)	(12,610)	(35,687)	(25,280)
Change in fair value of contingent consideration liabilities	2,238	(4,371)	(520)	(4,287)
Operating income	112,916	78,580	193,653	198,067
Interest expense	(8,654)	(15,821)	(22,660)	(29,697)
Interest income	275	267	1,034	576
Other income (expense), net	3,247	6,521	(6,580)	65,480
Income before income taxes	107,784	69,547	165,447	234,426
Provision for income taxes	32,989	41,088	49,149	84,932
Net income	74,795	28,459	116,298	149,494
Less: Net income attributable to non-controlling interests	849	1,115	3,666	1,662
Net income attributable to common stock	$ 73,946	$ 27,344	$ 112,632	$ 147,832
Earnings per share attributable to common stock:
Basic (in dollars per share)	$ 0.53	$ 0.19	$ 0.79	$ 1.02
Diluted (in dollars per share)	$ 0.52	$ 0.19	$ 0.78	$ 1.01
Shares used in computing earnings per share:
Weighted average basic shares outstanding (in shares)	139,856	145,935	142,154	145,525
Weighted average diluted shares outstanding (in shares)	143,521	147,241	145,213	146,865
Cash dividends declared per common share (in dollars per share)	$ 0.05	$ 0.04	$ 0.10	$ 0.08