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Revenues, Contract Assets and Contract Liabilities - Schedule of Contract Assets and Liabilities (Details) - USD ($)
$ in Thousands
Sep. 30, 2019
Dec. 31, 2018
Operating revenues:    
Contract assets - current $ 43,384 $ 46,018
Contract assets - non-current 9,104 6,986
Contract liabilities - current (12,750) (16,789)
Contract liabilities - non-current (177) (1,436)
Net contract assets $ 39,561 $ 34,779