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Debt (Committed Funding Facilities) (Details) - Committed funding facilities - Secured debt - USD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Debt Instrument [Line Items]    
Long-term line of credit $ 0 $ 450
Line of credit facility, remaining borrowing capacity 560 2,050
Line of credit facility, maximum borrowing capacity 560 2,500
Revolving credit facility    
Debt Instrument [Line Items]    
Line of credit facility, maximum borrowing capacity 560  
Revolving secured funding facilities 1 year or greater    
Debt Instrument [Line Items]    
Line of credit facility, maximum borrowing capacity 200  
Ally Financial Inc    
Debt Instrument [Line Items]    
Long-term line of credit 0 450
Line of credit facility, remaining borrowing capacity 560 2,050
Line of credit facility, maximum borrowing capacity   $ 2,500
Ally Financial Inc | Revolving credit facility    
Debt Instrument [Line Items]    
Line of credit facility, maximum borrowing capacity $ 560