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Business Acquisitions, Goodwill and Intangible Assets - Intangible Assets (Details) - USD ($)
$ in Thousands
6 Months Ended
Mar. 31, 2018
Sep. 30, 2017
Identifiable intangible assets with finite useful lives    
Gross Amount $ 1,305,800 $ 1,301,900
Accumulated Amortization (937,600) (886,800)
Intangible Assets, Net 368,215 415,096
Backlog and customer relationships    
Identifiable intangible assets with finite useful lives    
Gross Amount 1,287,500 1,283,600
Accumulated Amortization (920,800) (870,200)
Intangible Assets, Net $ 366,700 413,400
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 (16,800) (16,600)
Intangible Assets, Net $ 1,500 $ 1,700
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