SUMMARIZED QUARTERLY FINANCIAL DATA (UNAUDITED) (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Data [Abstract]
Total revenue		$ 522,071	$ 493,593	$ 493,986	$ 499,065	$ 524,677	$ 493,190	$ 488,244	$ 491,914	$ 2,008,715	$ 1,998,025	$ 1,965,556
Gross profit		315,010	289,870	291,458	299,442	309,522	295,556	298,043	303,156	1,195,780	1,206,277	1,222,849
Net income (loss)		$ 44,824	$ (318,493)	$ 32,582	$ 41,190	$ 57,170	$ (31,083)	$ 60,207	$ 63,336	$ (199,897)	$ 149,630	$ 230,155
Earnings (loss) per share:
(Loss) earnings per share - basic		$ 1.06	$ (7.49)	$ 0.75	$ 0.93	$ 1.25	$ (0.67)	$ 1.26	$ 1.32	$ (4.65)	$ 3.18	$ 4.75
(Loss) earnings per share - diluted		1.06	(7.49)	0.75	0.93	1.25	(0.67)	1.25	1.31	(4.65)	3.16	4.72
Cash dividends per share		$ 0.30	$ 0.30	$ 0.30	$ 0.30	$ 0.30	$ 0.30	$ 0.30	$ 0.30	$ 1.20	$ 1.20	$ 1.20
Items affecting comparability of quarterly results
Asset impairment charges		$ 0	$ 390,980	$ 0	$ 0	$ 0	$ 99,170	$ 0	$ 2,149	$ 390,980	$ 101,319	$ 54,880
Restructuring and integration expense		23,356	27,674	17,497	6,283	7,406	5,104	6,371	2,322	74,810	21,203	9,130
Gain on sales of businesses and customer lists						2,786	1,765	3,862	7,228	(124)	15,641	8,703
Certain legal-related expense		3	0	6,005	412	7,769	1,805	631	297
CEO transition costs		851	1,145	1,906	5,488	3,058	2,622	1,530	0	$ 9,390	7,210
Impact of Tax Cuts and Jobs Act						(582)	(1,249)	441	(310)		$ (1,700)	$ (20,500)
Other discrete income tax (benefit) expense	[1]	$ (298)	$ 62,854	$ 1,194	$ 926	$ (1,987)	$ 15,634	$ (1,167)	$ (579)

[1] Relates primarily to the tax effects of share-based compensation and the non-deductible portion of goodwill impairment charges in the third quarter of each year.