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Basis of Presentation, Organization and Recent Accounting Pronouncements - Additional Information (Details) - USD ($)
$ / shares in Units, $ in Millions
3 Months Ended
Nov. 24, 2019
Dec. 31, 2019
Sep. 30, 2019
New Accounting Pronouncements or Change in Accounting Principle [Line Items]      
Common stock, par value (in dollars per share)   $ 0.01 $ 0.01
Merger Agreement [Member]      
New Accounting Pronouncements or Change in Accounting Principle [Line Items]      
Common stock, par value (in dollars per share) $ 0.01    
Costs related to merger   $ 25  
The Charles Schwab Corporation [Member] | Merger Agreement [Member]      
New Accounting Pronouncements or Change in Accounting Principle [Line Items]      
Common stock, par value (in dollars per share) $ 0.01    
Number of shares to be received upon conversion (in shares) 1.0837    
Percent threshold of shares issued 9.90%