Revenues, Contract Assets and Contract Liabilities - Schedule of Contract Assets (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Operating revenues:
Unbilled contract receivables, gross	$ 27,644	$ 21,703
Retainage	31,537	31,301
Total contract assets	59,181	53,004
Less: current portion	50,580	46,018
Contract Assets - non-current	$ 8,601	$ 6,986

Amount billed to customers under long-term contracts or programs that have been withheld because of retainage provisions in a contract.

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