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Discontinued Operations, Goodwill, and Intangible Assets - Intangible Assets (Details) - USD ($)
$ in Thousands
12 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Identifiable intangible assets with finite useful lives    
Intangible Assets, Net $ 76,917 $ 99,636
Backlog and customer relationships    
Identifiable intangible assets with finite useful lives    
Gross Amount 662,800 661,400
Accumulated Amortization (585,900) (561,800)
Intangible Assets, Net $ 76,900 $ 99,600
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