SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Allowances for credit loss rollforward (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accounts Receivable, Allowance for Credit Loss
Balance at beginning of year	$ 5,738	$ 4,647	$ 5,427
Provisions for losses and sales adjustments - accounts receivable	3,517	1,636	(569)
Collections of accounts previously written off	0	(545)	(211)
Balance at end of year	$ 9,255	$ 5,738	$ 4,647