SUPPLEMENTAL BALANCE SHEET AND CASH FLOW INFORMATION (trade accounts receivable) (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2020	Dec. 31, 2019
Trade accounts receivable
Trade accounts receivable - gross					$ 168,387	$ 168,406
Allowance for uncollectible accounts		$ (6,428)	$ (3,639)	$ (3,639)	(6,428)	(4,985)
Trade accounts receivable - net	[1]				161,959	163,421
Unbilled receivables					$ 21,319	$ 17,925
Allowance for uncollectible accounts
Balance, beginning of year		4,985	3,639	2,884
Bad debt expense		5,003	5,213	3,622
Write-offs, net of recoveries		(3,560)	(3,867)	(2,867)
Balance, end of year		$ 6,428	$ 4,985	$ 3,639

[1] Includes unbilled receivables of $21,319 as of December 31, 2020 and $17,925 as of December 31, 2019.