ACQUISITIONS - Narrative (Details) - USD ($) $ in Thousands			3 Months Ended		7 Months Ended	9 Months Ended
	Mar. 10, 2021	Jan. 03, 2020	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2021	Sep. 30, 2020	Mar. 31, 2021	Dec. 31, 2020
Business Acquisition [Line Items]
Contingent consideration related to acquisition			$ 24,129		$ 24,129	$ 24,129			$ 0
Consolidated net revenue			138,562	$ 127,253		441,496	$ 426,339
Operating expense			(89,132)	$ (51,511)		(102,927)	(167,611)
Goodwill increase (decrease)						(4,480)
Goodwill			175,973		175,973	175,973			$ 136,091
Flexiti
Business Acquisition [Line Items]
Equity interests acquired	100.00%
Payments to acquire businesses, gross	$ 86,500
Debt costs in conjunction with the acquisition	6,300
Contingent consideration related to acquisition	20,600		24,100		24,100	24,100		$ 20,600
Consolidated net revenue			3,100			7,900
Operating expense			19,600			36,500
Goodwill increase (decrease)					(4,480)	(4,500)
Goodwill	44,901		40,421		40,421	40,421
Business combination, consideration transferred, equity interests issued and issuable	8,100
Business combination, consideration transferred, equity interests issued relates to RSU contingent consideration	$ 4,000
Business acquisition, service period term	2 years
Business acquisition, incurred costs related to this acquisition			$ 3,400		$ 3,400	3,400
Payments to acquire businesses, net of cash acquired						91,203	0
Flexiti | Minimum
Business Acquisition [Line Items]
Contingent consideration related to acquisition	$ 0
Flexiti | Maximum
Business Acquisition [Line Items]
Contingent consideration related to acquisition	$ 32,800
Ad Astra
Business Acquisition [Line Items]
Equity interests acquired		100.00%
Operating expense						2,600
Goodwill		$ 14,791
Payments to acquire businesses, net of cash acquired		14,400				$ 0	$ 14,418
Goodwill, estimated amount tax-deductible		$ 15,400