8. Receivables, net

(in thousands)	September 30, 2018		December 31, 2017		September 30, 2017
Contracts completed and in progress:
Billed	$	351,802	$	252,467	$	321,851
Unbilled		147,950		77,135		147,565
Retentions		—		91,135		88,650
Total contracts completed and in progress		499,752		420,737		558,066
Material sales		73,282		42,192		58,920
Other		45,125		17,014		10,412
Total gross receivables		618,159		479,943		627,398
Less: allowance for doubtful accounts		89		152		317
Total net receivables	$	618,070	$	479,791	$	627,081

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 September 30, 2018, December 31, 2017 and September 30, 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. As of September 30, 2018, December 31, 2017 and September 30, 2017, the estimated recovery from back charge claims included in Other receivables was $1.4 million, $1.1 million and $0.3 million, respectively.

Certain construction contracts include retainage provisions that were included in contract assets as of September 30, 2018 and in receivables, net as of December 31, 2017 and September 30, 2017 in our condensed consolidated balance sheets. As of September 30, 2018, December 31, 2017 and September 30, 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.