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Debt (Pledged Assets Related to Secured Borrowings and Repurchase Agreement) (Details) - USD ($)
$ in Millions
Sep. 30, 2021
Dec. 31, 2020
Pledged Assets related to secured borrowings [Line Items]    
Pledged assets, mortgage finance receivables $ 16,378 $ 14,979
Pledged assets, restricted as collateral 26,777 35,798
Secured debt 6,091 9,992
Consumer | Automotive loan    
Pledged Assets related to secured borrowings [Line Items]    
Pledged assets, finance receivables 10,389 9,953
Commercial    
Pledged Assets related to secured borrowings [Line Items]    
Pledged assets, finance receivables 10 10,866
Ally Bank    
Pledged Assets related to secured borrowings [Line Items]    
Pledged assets, mortgage finance receivables 16,378 14,979
Pledged assets, restricted as collateral 26,664 35,355
Secured debt 6,008 9,634
Ally Bank | Pledged assets for Federal Home Loan Bank    
Pledged Assets related to secured borrowings [Line Items]    
Pledged assets, restricted as collateral 16,400 20,000
Ally Bank | Pledged assets for Federal Reserve Bank    
Pledged Assets related to secured borrowings [Line Items]    
Pledged assets, restricted as collateral 2,400 2,400
Ally Bank | Consumer | Automotive loan    
Pledged Assets related to secured borrowings [Line Items]    
Pledged assets, finance receivables 10,276 9,510
Ally Bank | Commercial    
Pledged Assets related to secured borrowings [Line Items]    
Pledged assets, finance receivables $ 10 $ 10,866