UNAUDITED QUARTERLY DATA (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
UNAUDITED QUARTERLY DATA
Revenues	$ 444,623	$ 487,739	$ 480,461	$ 408,742	$ 414,713	$ 450,442	$ 433,555	$ 375,247	$ 1,821,565	$ 1,673,957	$ 1,573,477
Gross profit (Revenues less cost of services provided)	223,389	251,452	249,689	202,599	207,194	231,661	229,075	186,084
Net Income	$ 50,968	$ 66,628	$ 65,542	$ 48,525	$ 33,735	$ 51,430	$ 53,689	$ 40,270	$ 231,663	$ 179,124	$ 167,369
Income per share:
Income per share - Basic (in dollars per share)	$ 0.16	$ 0.20	$ 0.20	$ 0.15	$ 0.10	$ 0.16	$ 0.17	$ 0.12	$ 0.71	$ 0.55	$ 0.51
Income per share - Diluted (in dollars per share)	$ 0.16	$ 0.20	$ 0.20	$ 0.15	$ 0.10	$ 0.16	$ 0.17	$ 0.12	$ 0.71	$ 0.55	$ 0.51