REVENUE - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Disaggregation of Revenue [Line Items]
Discounts and incentives	$ 5,000	$ 2,800	$ 12,500	$ 6,400
Gross receivables from members and customers	60,500		60,500		$ 38,600
Undeposited funds	14,000		14,000		14,100
Allowance for expected credit losses	5,500		5,500		2,300
Revenue	301,978	194,781	849,215	485,208
Capitalized sales commission and referral fees	3,400	100	7,400	2,200
Capitalized contract cost, amortization	2,400	$ 200	5,900	$ 3,900
Prepaid Expenses and Other Current Assets
Disaggregation of Revenue [Line Items]
Capitalized contract cost, net	6,200		6,200		5,000
Other Noncurrent Assets
Disaggregation of Revenue [Line Items]
Capitalized contract cost, net	$ 1,000		$ 1,000		$ 800