UNAUDITED QUARTERLY DATA (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
UNAUDITED QUARTERLY DATA
Revenues	$ 306,393	$ 340,179	$ 334,872	$ 289,465	$ 289,056	$ 323,929	$ 320,436	$ 271,643	$ 1,270,909	$ 1,205,064	$ 1,136,890
Gross profit (Revenues less cost of services provided)	143,368	169,701	168,879	141,383	137,854	158,832	160,791	130,745
Net Income	$ 22,914	$ 32,211	$ 33,127	$ 23,080	$ 21,595	$ 29,415	$ 31,061	$ 18,640	$ 111,332	$ 100,711	$ 90,002
Income per share:
Income per share Basic (in dollars per share)	$ 0.16	$ 0.22	$ 0.23	$ 0.16	$ 0.15	$ 0.20	$ 0.21	$ 0.13	$ 0.76	$ 0.69	$ 0.61
Income per share Diluted (in dollars per share)	$ 0.16	$ 0.22	$ 0.23	$ 0.16	$ 0.15	$ 0.20	$ 0.21	$ 0.13	$ 0.76	$ 0.69	$ 0.61