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Summary of Significant Accounting Policies - Goodwill and Intangible Assets (Details) - USD ($)
1 Months Ended 12 Months Ended
Apr. 30, 2018
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2018
Finite-Lived Intangible Assets [Line Items]        
Useful life       3 years
Impairment of intangible assets, finite-lived   $ 0 $ 0 $ 75,000,000
Finite-lived trademarks and tradenames, gross       8,000,000
Accumulated impairment losses   0 0  
Provision for goodwill or other intangible asset impairments   $ 0 $ 0 $ 0
Trademarks and tradenames        
Finite-Lived Intangible Assets [Line Items]        
Useful life 3 years      
Customer relationships, noncompete agreements, and other        
Finite-Lived Intangible Assets [Line Items]        
Useful life   15 years    
CMG | Customer Relationships        
Finite-Lived Intangible Assets [Line Items]        
Useful life   35 years