Consolidated Statements of Operations - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Statement [Abstract]
Revenues	$ 12,112,153	$ 11,171,423	$ 9,466,478
Cost of services (including depreciation)	10,511,901	9,691,459	8,224,618
Gross profit	1,600,252	1,479,964	1,241,860
Selling, general and administrative expenses	955,991	857,574	777,920
Amortization of intangible assets	62,091	43,994	32,205
Asset impairment charges	13,892	49,375	58,057
Change in fair value of contingent consideration liabilities	13,404	(11,248)	(5,171)
Operating income	554,874	540,269	378,849
Interest expense	(66,890)	(36,945)	(20,946)
Interest income	927	1,555	832
Other income (expense), net	83,376	(47,213)	(4,978)
Income before income taxes	572,287	457,666	353,757
Provision for income taxes	165,472	161,659	35,532
Net income	406,815	296,007	318,225
Less: Net income attributable to non-controlling interests	4,771	2,661	3,247
Net income attributable to common stock	$ 402,044	$ 293,346	$ 314,978
Earnings per share attributable to common stock:
Basic (in dollars per share)	$ 2.76	$ 1.92	$ 2.02
Diluted (in dollars per share)	$ 2.73	$ 1.90	$ 2.00
Shares used in computing earnings per share:
Weighted average basic shares outstanding (in shares)	145,710	152,963	156,124
Weighted average diluted shares outstanding (in shares)	147,534	154,226	157,155
Cash dividends declared per share (in dollars per share)	$ 0.17	$ 0.04	$ 0