8. Receivables, net

(in thousands)	June 30, 2018		December 31, 2017		June 30, 2017
Construction contracts completed and in progress:
Billed	$	340,548	$	252,467	$	208,635
Unbilled		71,464		77,135		135,072
Retentions		—		91,135		75,891
Total construction contracts completed and in progress		412,012		420,737		419,598
Construction material sales		64,128		42,192		54,165
Other		16,644		17,014		10,892
Total gross receivables		492,784		479,943		484,655
Less: allowance for doubtful accounts		66		152		410
Total net receivables	$	492,718	$	479,791	$	484,245

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 June 30, 2018, December 31, 2017 and June 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 June 30, 2018, December 31, 2017 and June 30, 2017, the estimated recovery from back charge claims included in Other receivables was $1.6 million, $1.1 million and $0.3 million, respectively.

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