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Discontinued Operations, Goodwill, and Intangible Assets - Intangible Assets (Details) - USD ($)
$ in Thousands
3 Months Ended
Dec. 31, 2020
Sep. 30, 2020
Identifiable intangible assets with finite useful lives    
Intangible Assets, Net $ 72,003 $ 76,917
Backlog and customer relationships    
Identifiable intangible assets with finite useful lives    
Gross Amount 663,300 662,800
Accumulated Amortization (591,300) (585,900)
Intangible Assets, Net $ 72,000 $ 76,900
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