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Stockholders' Equity - Additional Information (Details) - USD ($)
 12 Months Ended
Jul. 20, 2018
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2018
Class of Stock [Line Items]          
Preferred stock, shares authorized   5,000,000 5,000,000    
Common stock, shares authorized   500,000,000 500,000,000    
Stock repurchased during period (in shares)   200,000 0 1,400,000  
Stock repurchased during period, value   $ 30,000,000   $ 115,000,000  
Accelerated Share Repurchases, Settlement (Payment) or Receipt $ 1,000,000,000        
Accelerated Share Repurchase Initial Delivery Percent of Total 80.00%        
Stock Repurchased and Retired During Period, Value   $ 30,000,000   $ 114,994,000  
Accelerated Share Repurchase Agreement          
Class of Stock [Line Items]          
Stock repurchased during period (in shares) 8,200,000     3,000,000.0  
Accelerated Share Repurchase Agreement | Final settlement          
Class of Stock [Line Items]          
Stock Repurchased and Retired During Period, Value         $ 200,000,000
Maximum          
Class of Stock [Line Items]          
Common stock, shares authorized   500,000,000      
Stock authorized to repurchase   $ 1,000,000,000      
Series A Junior Participating Preferred Stock          
Class of Stock [Line Items]          
Preferred stock, shares authorized   500,000