7.	Receivables, net

(in thousands)	March 31, 2018		December 31, 2017		March 31, 2017
Construction contracts completed and in progress:
Billed	$	214,494	$	252,467	$	142,216
Unbilled		73,128		77,135		85,098
Retentions		—		91,135		82,692
Total construction contracts completed and in progress		287,622		420,737		310,006
Construction material sales		28,233		42,192		25,582
Other		14,442		17,014		15,977
Total gross receivables		330,297		479,943		351,565
Less: allowance for doubtful accounts		105		152		474
Total net receivables	$	330,192	$	479,791	$	351,091

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 March 31, 2018, December 31, 2017 and March 31, 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 March 31, 2018, December 31, 2017 and March 31, 2017, the estimated recovery from back charge claims included in Other receivables was $1.2 million, $1.1 million and $0.3 million, respectively.

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