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Revenue - Contract assets (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Revenue    
Unbilled revenue $ 251,429 $ 249,577
Retention receivable 81,393 88,953
Contract materials (not yet installed) 11,984 25,715
Contract assets 344,806 $ 364,245
Increase (decrease) in contract assets $ 19,400