Note 14—Customer Concentrations

We operate in multiple industry segments encompassing the construction of commercial, industrial, and public works infrastructure assets primarily throughout the United States. During the years ended December 31, 2018, 2017 and 2016, we generated 35.3%, 38.4% and 45.6%, of our revenue, respectively, from the following customers (in thousands):


		2018			2017			2016
Description of Customer's Business	Segment	Amount		Percentage	Amount		Percentage	Amount		Percentage
Public gas and electric utility	Utilities/Power	$	250,286	8.5%	$	210,747	8.9%	$	184,002	9.2%
Private gas and electric utility	Utilities		232,162	7.9%		190,659	8.0%		201,443	10.1%
State DOT	Civil		202,452	6.9%		222,142	9.3%		193,049	9.7%
Pipeline operator	Pipeline		198,198	6.7%		*	*		*	*
Pipeline operator	Pipeline/Power		155,280	5.3%		*	*		*	*
Chemical/Energy producer	Power/Civil		*	*		160,995	6.8%		208,458	10.4%
Pipeline operator	Pipeline		*	*		128,182	5.4%		*	*
Pipeline operator	Pipeline		*	*		*	*		123,055	6.2%
		$	1,038,378	35.3%	$	912,725	38.4%	$	910,007	45.6%

(*)Indicates a customer with less than 5.0% of revenue during such period.

Typically, the top ten customers in any one calendar year generate revenue in excess of 50.0% of total revenue and consist of a different group of customers in each year. For the years ended December 31, 2018, 2017 and 2016, approximately 52.2%, 56.4% and 60.4%, respectively, of total revenue were generated from our top ten customers in that year. In each of the years, a different group of customers comprised the top ten customers by revenue.

At December 31, 2018, approximately 9.3% of our accounts receivable were due from one customer, and that customer provided 5.3% of our revenue for the year ended December 31, 2018. At December 31, 2017, approximately 4.3% of our accounts receivable were due from one customer, and that customer provided 8.9% of our revenue for the year ended December 31, 2017.

On January 29, 2019, one of our utility customers filed for reorganization under Chapter 11 of the U.S. Bankruptcy Code. As of December 31, 2018, the utility customer comprised approximately 1.9% of our total accounts receivable. In addition to accounts receivable, there is approximately $36.0 million in unbilled revenue, net as of December 31, 2018. For the year ended December 31, 2018, the customer accounted for approximately 8.5% of our total revenue. Our exposure to diverse end markets limits the potential for any one client or job to have a material adverse impact on our operations. Although we do not currently expect a material impact to our financial performance as a result of this customer’s recent announcement, the failure to recover amounts due to us from this customer or any customer who enters bankruptcy could have a negative impact on our results of operations and cash flows, and the loss of a customer due to bankruptcy could have a negative impact on our financial condition, results of operations and cash flows. We do not believe a reserve for the accounts receivable and unbilled revenue is appropriate at this time. However, we will closely monitor our current and future potential exposure.