UNAUDITED QUARTERLY DATA (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
UNAUDITED QUARTERLY DATA
Revenues	$ 324,707	$ 362,155	$ 350,798	$ 299,714	$ 306,393	$ 340,179	$ 334,872	$ 289,465	$ 1,337,374	$ 1,270,909	$ 1,205,064
Gross profit (Revenues less cost of services provided)	157,350	181,020	176,437	144,108	143,368	169,701	168,879	141,383
Net Income	$ 27,957	$ 36,200	$ 35,994	$ 23,179	$ 22,914	$ 32,211	$ 33,127	$ 23,080	$ 123,330	$ 111,332	$ 100,711
Income per share:
Income per share Basic (in dollars per share)	$ 0.19	$ 0.25	$ 0.25	$ 0.16	$ 0.16	$ 0.22	$ 0.23	$ 0.16	$ 0.84	$ 0.76	$ 0.69
Income per share Diluted (in dollars per share)	$ 0.19	$ 0.25	$ 0.25	$ 0.16	$ 0.16	$ 0.22	$ 0.23	$ 0.16	$ 0.84	$ 0.76	$ 0.69