EQUITY-BASED COMPENSATION PLANS (NARRATIVE) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Dec. 31, 2019
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Equity-based compensation expense	$ 4,600	$ 3,500	$ 11,969	$ 9,471
Minimum
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Share-based Compensation Award Vesting Period			4 years
Maximum
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]
Equity-based compensation expense			$ 4,100	$ 10,000
Unrecognized Compensation Expense	37,300		$ 37,300
Non-option Equity Awards Granted			219,883	539,697
Performance Shares [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Equity-based compensation expense	300	400	$ 800	$ 1,100
Share-based Compensation Arrangement by Share-based Payment Award, Description						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.
Performance Shares [Member] | 2020 PRSUs [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Equity-based compensation expense	200		1,200
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, the annual value of the PRSUs issued will be between $0.2 million and $2.0 million and 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. The Company recognized compensation expense related to PRSUs of $0.2 million and $1.2 million, respectively, for the three and nine months ended September 30, 2021.
Performance Shares [Member] | Minimum | 2019 PRSUs [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Equity-based compensation expense						$ 400
Performance Shares [Member] | Minimum | 2020 PRSUs [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Equity-based compensation expense					$ 200
Performance Shares [Member] | Maximum | 2019 PRSUs [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Equity-based compensation expense						$ 1,400
Performance Shares [Member] | Maximum | 2020 PRSUs [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Equity-based compensation expense					$ 2,000
Cost of Services
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Equity-based compensation expense	2,000	1,300	4,400	3,400
Selling General And Administrative Expenses [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Equity-based compensation expense	$ 2,600	$ 2,200	$ 7,600	$ 6,100