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Revenues, Contract Assets and Contract Liabilities - Schedule of Contract Assets (Details) - USD ($)
$ in Thousands
Sep. 30, 2019
Dec. 31, 2018
Operating revenues:    
Unbilled contract receivables, gross $ 22,813 $ 21,703
Retainage 29,675 31,301
Total contract assets 52,488 53,004
Less: current portion 43,384 46,018
Contract Assets - non-current $ 9,104 $ 6,986