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EMPLOYEE COMPENSATION PLANS (NARRATIVE) (Details) - USD ($) $ / shares in Units, $ in Thousands, shares in Millions	1 Months Ended	12 Months Ended
	Feb. 29, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Defined Contribution Plan, Maximum Annual Contributions Per Employee, Percent		75.00%
Company Match on US Defined Contribution Plans		$ 4,200	$ 6,700	$ 5,200
Share-based Compensation Award Vesting Period		1 year
Share-based Compensation Arrangement by Share-based Payment Award, Description		During 2020, the Company awarded PRSUs that are subject to service and performance vesting conditions. If defined minimum targets are met, Company shares will be issued that vest immediately. If the defined minimum targets are not met, then no shares will be issued. The number of shares awarded are based on the Company’s annual revenue and adjusted operating income for the fiscal years 2021 and 2022. Each fiscal year’s revenue and adjusted operating income will determine the award amount. Expense for these awards will begin at the start of the requisite service period, beginning January 1, 2021.	During 2019, the Company awarded performance restricted stock units (“PRSUs”) that are subject to service and performance vesting conditions. If defined minimum targets are met, the annual value of the PRSUs issued will be between $0.4 million and $1.4 million and vest immediately. If the defined minimum targets are not met, then no shares will be issued. The award amounts are based on the Company’s annual adjusted operating income for the fiscal years 2019, 2020 and 2021. Each fiscal year’s adjusted operating income will determine the award amount.
Equity-based compensation expense		$ 12,507	$ 12,814	12,145
Total Tax Benefit Recognized for Equity-Based Compensation Expense		$ 3,500	4,200	3,700
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Grants In Period Weighted Average Grant Date Fair Value		$ 44.70
Intrinsic Value of Non-Vested RSUs		$ 75,100
Proceeds from exercise of stock options		0	0	208
2020 Plan [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Shares Available for Issuance under Current Equity Compensation Plan	4.0
Cost of Services
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Equity-based compensation expense		4,300	4,700	4,700
Selling General And Administrative Expenses
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Equity-based compensation expense		8,200	$ 8,100	$ 7,400
Minimum \| 2020 Plan [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Share-based Compensation Award Vesting Period	3 years
Maximum \| 2020 Plan [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Share-based Compensation Award Vesting Period	5 years
Restricted Stock Units (RSUs) [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Unrecognized Compensation Expense		$ 25,700
Requisite Service Period		1 year 7 months 6 days
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Grants In Period Weighted Average Grant Date Fair Value		$ 44.70	$ 40.10	$ 35.15
Intrinsic Value of RSUs Vested		$ 11,500	$ 12,500	$ 12,500
Performance Shares [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Equity-based compensation expense		$ 1,100	$ 1,400