ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (INCOME STATEMENT CLASSIFICATION TABLE) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Presentation of Income Statement Reclassifications [Line Items]
Sales Revenue Services Net	$ 570,974	$ 492,980	$ 453,081	$ 432,213	$ 461,326	$ 395,507	$ 392,515	$ 394,356	$ 1,949,248	$ 1,643,704	$ 1,509,171
Benefit from (provision for) income taxes	$ (11,284)	$ (8,415)	$ (11,039)	$ (10,199)	$ (5,670)	$ (5,196)	$ (7,345)	$ (7,466)	(40,937)	(25,677)	(16,483)
Net income									129,331	84,734	39,755
Accumulated Other Comprehensive Income (Loss)
Presentation of Income Statement Reclassifications [Line Items]
Net Income (Loss) - Other									(21,019)	3,283	13,214
Foreign Currency Translation Adjustment
Presentation of Income Statement Reclassifications [Line Items]
Net Income (Loss) - Other									(19,619)
Tax effect
Presentation of Income Statement Reclassifications [Line Items]
Provision for income taxes - Other									60	23	42
Derivative Valuation, Net of Tax
Presentation of Income Statement Reclassifications [Line Items]
Net Income (Loss) - Other									(1,928)	3,085	12,810
Cost of Services
Presentation of Income Statement Reclassifications [Line Items]
Cost of services									(588)	(221)	(446)
Reclassification from accumulated other comprehensive income
Presentation of Income Statement Reclassifications [Line Items]
Net Income (Loss) - Other									(528)	(198)	(404)
Reclassification from accumulated other comprehensive income | Tax effect
Presentation of Income Statement Reclassifications [Line Items]
Benefit from (provision for) income taxes									(690)	1,143	4,738
Reclassification from accumulated other comprehensive income | Derivative Valuation, Net of Tax
Presentation of Income Statement Reclassifications [Line Items]
Net income									1,928	(3,085)	(12,810)
Foreign Exchange Forward | Reclassification from accumulated other comprehensive income | Foreign Currency Translation Adjustment
Presentation of Income Statement Reclassifications [Line Items]
Sales Revenue Services Net									$ 2,618	$ (4,228)	$ (17,548)