RECEIVABLES

A summary of accounts receivable as of December 31, 2019 and 2018 is as follows (in thousands):



	December 31,
	2019			2018
Trade accounts receivable	$	192,743		$	207,266
Unbilled revenues	62,864			76,268
Allowance for doubtful accounts	(9,990		)	(15,182		)
Total	$	245,617		$	268,352

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,
	2019			2018			2017
Balance at beginning of period	$	15,182		$	11,308		$	7,835
Provision for doubtful accounts	(2,573		)	11,662			7,097
Write-off of bad debts	(2,619		)	(7,788		)	(3,624		)
Balance at end of period	$	9,990		$	15,182		$	11,308