SUMMARY OF QUARTERLY RESULTS (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Accrued payroll liability error in prior period	$ 31,091				$ 24,958				$ 31,091	$ 24,958
Total revenue	618,201	$ 672,244	$ 678,520	$ 670,247	607,156	$ 656,326	$ 675,216	$ 674,456	2,639,212	2,613,154	$ 2,626,761
Gross profit	228,234	250,644	256,332	249,433	220,193	247,748	258,580	253,719	984,643	980,240	990,463
Operating income	83,714	82,150	117,638	109,605	88,481	108,725	121,051	121,255	393,107	439,512	460,498
Net income	$ 42,922	$ 45,750	$ 67,357	$ 63,270	$ 51,768	$ 64,314	$ 69,887	$ 69,903	$ 219,299	$ 255,872	$ 265,021
Weighted average shares outstanding:
Basic (in shares)	78,775	83,669	85,501	87,865	88,824	89,814	90,414	92,975	83,927	90,493	96,481
Diluted (in shares)	79,008	83,958	85,777	88,105	89,044	90,233	90,931	93,684	84,186	90,918	97,383
Earnings per share:
Basic (in usd per share)	$ 0.54	$ 0.55	$ 0.79	$ 0.72	$ 0.58	$ 0.72	$ 0.77	$ 0.75	$ 2.61	$ 2.83	$ 2.75
Diluted (in usd per share)	$ 0.54	$ 0.54	$ 0.79	$ 0.72	$ 0.58	$ 0.71	$ 0.77	$ 0.75	$ 2.60	$ 2.81	$ 2.72
Immaterial Error Correction
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Accrued payroll liability error in prior period				$ 2,800