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Revenue - Contract assets (Details) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Jun. 30, 2018
Dec. 31, 2018
Dec. 31, 2017
Unbilled revenue   $ 249,577 $ 160,092
Retention receivable   88,953 66,586
Contract materials (not yet installed)   25,715 39,224
Contract assets   364,245 $ 265,902
Increase in contract assets   $ 98,300  
Willbros Group, Inc      
Increase in contract assets $ 30,800