UNAUDITED QUARTERLY DATA (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 505,985	$ 556,466	$ 523,957	$ 429,069	$ 444,623	$ 487,739	$ 480,461	$ 408,742	$ 2,015,477	$ 1,821,565	$ 1,673,957
Gross profit (Revenues less cost of services provided)	251,701	287,748	270,624	211,811	223,389	251,452	249,689	202,599
Net Income	$ 50,762	$ 44,064	$ 64,295	$ 44,226	$ 50,968	$ 66,628	$ 65,542	$ 48,525	$ 203,347	$ 231,663	$ 179,124
Income per share:
Income per share-Basic	$ 0.16	$ 0.13	$ 0.20	$ 0.14	$ 0.16	$ 0.20	$ 0.20	$ 0.15	$ 0.62	$ 0.71	$ 0.55
Income per share-Diluted	$ 0.16	$ 0.13	$ 0.20	$ 0.14	$ 0.16	$ 0.20	$ 0.20	$ 0.15	$ 0.62	$ 0.71	$ 0.55