RECEIVABLES (Tables)

12 Months Ended

Dec. 31, 2018

Receivables [Abstract]

Summary of Accounts Receivable

A summary of accounts receivable as of December 31, 2018 and 2017 is as follows (in thousands):

	December 31,
	2018			2017
Trade accounts receivable	$	207,266		$	244,133
Unbilled revenues	76,268			69,138
Allowance for doubtful accounts	(15,182		)	(11,308		)
Total	$	268,352		$	301,963

Summary of Activity in Allowance for Doubtful Accounts

The following summarizes the activity in the allowance for doubtful accounts (in thousands):

	Twelve Months Ended December 31,
	2018			2017			2016
Balance at beginning of period	$	11,308		$	7,835		$	3,548
Provision for doubtful accounts	11,662			7,097			6,336
Write-off of bad debts	(7,788		)	(3,624		)	(2,049		)
Balance at end of period	$	15,182		$	11,308		$	7,835