UNAUDITED QUARTERLY DATA (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
UNAUDITED QUARTERLY DATA
Revenues	$ 414,713	$ 450,442	$ 433,555	$ 375,247	$ 385,614	$ 423,994	$ 411,133	$ 352,736	$ 1,673,957	$ 1,573,477	$ 1,485,305
Gross profit (Revenues less cost of services provided)	207,194	231,661	229,075	186,084	192,619	218,386	215,190	174,934
Net Income	$ 33,735	$ 51,430	$ 53,689	$ 40,270	$ 38,007	$ 49,651	$ 47,783	$ 31,928	$ 179,124	$ 167,369	$ 152,149	$ 137,664
Income per share:
Income per share—Basic (in dollars per share)	$ 0.15	$ 0.24	$ 0.25	$ 0.18	$ 0.17	$ 0.23	$ 0.22	$ 0.15	$ 0.82	$ 0.77	$ 0.70	$ 0.63
Income per share—Diluted (in dollars per share)	$ 0.15	$ 0.24	$ 0.25	$ 0.18	$ 0.17	$ 0.23	$ 0.22	$ 0.15	$ 0.82	$ 0.77	$ 0.70	$ 0.63