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Business Acquisitions, Goodwill and Intangible Assets - Intangible Assets (Details) - USD ($)
$ in Thousands
12 Months Ended
Sep. 30, 2018
Dec. 31, 2018
Identifiable intangible assets with finite useful lives    
Gross Amount $ 1,303,400 $ 1,301,800
Accumulated Amortization (983,500) (1,005,100)
Intangible Assets, Net 319,892 296,711
Backlog and customer relationships    
Identifiable intangible assets with finite useful lives    
Gross Amount 1,285,100 1,283,500
Accumulated Amortization (966,000) (987,400)
Intangible Assets, Net $ 319,100 296,100
Backlog and customer relationships | Minimum    
Identifiable intangible assets with finite useful lives    
Amortization Period 1 year  
Backlog and customer relationships | Maximum    
Identifiable intangible assets with finite useful lives    
Amortization Period 11 years  
Trademark / tradename    
Identifiable intangible assets with finite useful lives    
Gross Amount $ 18,300 18,300
Accumulated Amortization (17,500) (17,700)
Intangible Assets, Net $ 800 $ 600
Trademark / tradename | Minimum    
Identifiable intangible assets with finite useful lives    
Amortization Period 3 months 18 days  
Trademark / tradename | Maximum    
Identifiable intangible assets with finite useful lives    
Amortization Period 2 years