SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Allowance for credit losses (Details) - Trade Accounts Receivable - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	$ 771	$ 757	$ 773
Provision for credit loss expense	5,716	1,664	1,703
Write-offs	(1,730)	(2,156)	(2,210)
Recoveries	489	506	491
Total ending allowance balance	$ 5,246	$ 771	$ 757