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Debt (Committed Funding Facilities) (Details) - Committed funding facilities - Secured debt - USD ($)
$ in Millions
Sep. 30, 2021
Dec. 31, 2020
Debt Instrument [Line Items]    
Long-term line of credit $ 0 $ 0
Line of credit facility, remaining borrowing capacity 75 560
Line of credit facility, maximum borrowing capacity 75 560
Revolving credit facility    
Debt Instrument [Line Items]    
Line of credit facility, maximum borrowing capacity 75  
Ally Financial Inc    
Debt Instrument [Line Items]    
Long-term line of credit 0 0
Line of credit facility, remaining borrowing capacity 75 560
Line of credit facility, maximum borrowing capacity $ 75 $ 560