December 31,	2018		2017
Contracts completed and in progress:
Billed	$	285,521	$	252,467
Unbilled		98,755		77,135
Retentions		—		91,135
Total contracts completed and in progress		384,276		420,737
Material sales		45,286		42,192
Other		44,195		17,014
Total gross receivables		473,757		479,943
Less: allowance for doubtful accounts		511		152
Total net receivables	$	473,246	$	479,791

Receivables include billed and unbilled amounts for services provided to clients for which we have an unconditional right to payment as of the end of the applicable period and do not bear interest. Included in other receivables at December 31, 2018 and 2017 were items such as estimated recovery from back charge claims, notes receivable, fuel tax refunds, receivables from vendors and income tax refunds. No such receivables individually exceeded 10% of total net receivables at any of these dates.

During the years ended December 31, 2018, 2017 and 2016, our largest volume customer, including both prime and subcontractor arrangements, was the California Department of Transportation (“Caltrans”). Revenue recognized from contracts with Caltrans during 2018, 2017 and 2016, represented $282.9 million (8.5% of total revenue), $281.7 million (9.4% of total revenue) and $222.4 million (8.8% of total revenue), respectively, which was primarily in the Transportation segment.

We regularly review our accounts receivable, including past due amounts, to determine their probability of collection. If it is probable that an amount is uncollectible, it is charged to bad debt expense and a corresponding reserve is established in allowance for doubtful accounts.

Certain construction contracts include retainage provisions that were included in contract assets as of December 31, 2018 and in receivables, net as of December 31, 2017 in our consolidated balance sheets. The balances billed but not paid by customers pursuant to these provisions generally become due upon completion and acceptance of the project work or products by the owners. As of December 31, 2018 and 2017, no retention receivable individually exceeded 10% of total net receivables at any of the presented dates. The majority of the retentions receivable are expected to be collected within one year and there were no retentions receivables determined to be uncollectible.