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Summary of Significant Accounting Policies - Goodwill (Details)
$ in Thousands
6 Months Ended
Jun. 30, 2019
USD ($)
division
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Goodwill And Intangible Assets [Line Items]      
Number of internal divisions | division 2    
Decrease in fair value of reporting units considered for impairment calculation 10.00%    
Goodwill $ 1,932,300 $ 1,899,879 $ 1,868,600
Intangible assets 264,284 280,180  
Pipeline and Industrial Infrastructure Services Division      
Goodwill And Intangible Assets [Line Items]      
Goodwill 589,694 $ 586,801 $ 596,073
Pipeline and Industrial Infrastructure Services Division | Material Handling Services and Midstream and Smaller-Scale Pipeline Market Reporting Units      
Goodwill And Intangible Assets [Line Items]      
Goodwill 49,000    
Intangible assets $ 9,700