SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Allowance for Doubtful Accounts (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020
Allowance for Doubtful Accounts
Beginning balance	$ 3,329	$ 3,221	$ 3,416	$ 2,962	$ 2,596	$ 2,406	$ 3,416	$ 2,406
Bad debt expense	1,441	466	504	946	898	539	2,411	2,383
Write-Offs, net of Recoveries	(138)	(358)	(699)	(800)	(532)	(349)
Ending balance	$ 4,632	$ 3,329	$ 3,221	$ 3,108	$ 2,962	$ 2,596	$ 4,632	$ 3,108