Accounts Receivable

3 Months Ended

Mar. 31, 2020

NOTE F—ACCOUNTS RECEIVABLE

Accounts receivable are recorded when billed or accrued and represent claims against third parties that will be settled in cash. The carrying value of our accounts receivable, net of the allowance for credit losses, represents their estimated net realizable value. At March 31, 2020 and December 31, 2019, accounts receivable (in thousands) consisted of the following:



	March 31, 2020			December 31, 2019
Trade receivables	$	193,372		$	178,182
Less: Allowance for credit losses	(9,599		)	(8,984		)
Net trade receivables	183,773			169,198
Other receivables⁽¹⁾	49,082			13,040
Total accounts receivable	$	232,855		$	182,238



(1	)	At March 31, 2020, other receivables included $16.1 million of refundable alternative minimum tax credits and $28 million of refunds attributable to the CARES Act provision for net operating loss ("NOL") carrybacks from 2014-2019. At December 31, 2019, other receivables included $8.1 million of refundable alternative minimum tax credits.

We classify our trade receivables into the following portfolio segments: Oil & Gas Proppants and Industrial & Speciality Products, which also aligns with our reporting segments. We estimate the allowance for credit losses based on historical collection trends, the age of outstanding receivables, risks attributable to specific customers, such as credit history, bankruptcy or other going concern issues, and current economic and industry conditions. If events or circumstances indicate that specific receivable balances may be impaired, further consideration is given to the collectability of those balances and the allowance is adjusted accordingly. Past due balances are written off when we have exhausted our internal and external collection efforts and have been unsuccessful in collecting the amount due.

The following table is a rollforward of the allowance for credit losses (in thousands) for the three months ended March 31, 2020 disaggregated by portfolio segments:



	Oil & Gas Proppants		Industrial & Specialty Products			Total
Beginning balance, December 31, 2019	$	7,640	$	1,344		$	8,984
Provision for credit losses	840		62			902
Write-offs	—		(287		)	(287		)
Ending balance, March 31, 2020	$	8,480	$	1,119		$	9,599

Our ten largest customers accounted for approximately 38% and 42% of total sales for the three months ended March 31, 2020 and 2019, respectively. Sales to one of our customers accounted for 10% and 12% of our total sales for the three months ended March 31, 2020 and 2019, respectively. No other customers accounted for 10% or more of our total sales. At March 31, 2020, one of our customer's accounts receivable represented 10% of our total trade accounts receivable. At December 31, 2019, the same customer's accounts receivable represented 12% of our total trade accounts receivable. At March 31, 2020, one other customer's accounts receivable represented 10% of our total trade accounts receivable. No other customers accounted for 10% or more of our total trade accounts receivable.