Receivables, Net

6 Months Ended

Jun. 30, 2019

Receivables [Abstract]

Receivables, Net

8.  Receivables, net

(in thousands)	June 30, 2019		December 31, 2018		June 30, 2018
Contracts completed and in progress:
Billed	$	313,185	$	285,521	$	340,548
Unbilled		163,950		98,755		71,464
Total contracts completed and in progress		477,135		384,276		412,012
Material sales		61,204		45,286		64,128
Other		14,260		44,195		16,644
Total gross receivables		552,599		473,757		492,784
Less: allowance for doubtful accounts		641		511		66
Total net receivables	$	551,958	$	473,246	$	492,718

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, 2019, December 31, 2018 and June 30, 2018 were items such as estimated recovery from back charge claims, notes receivable, fuel tax refunds and income tax refunds. No such receivables individually exceeded 10% of total net receivables at any of these dates.