RECEIVABLES

12 Months Ended

Dec. 31, 2018

Receivables [Abstract]

RECEIVABLES

RECEIVABLES

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

The allowance for doubtful accounts is our best estimate of the amount of probable credit losses in our existing accounts receivable. Account balances are charged off against the allowance after all means of collection have been exhausted and the potential for recovery is remote. 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