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Debt (Pledged Assets Related to Secured Borrowings and Repurchase Agreement) (Details) - USD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Pledged Assets related to secured borrowings [Line Items]    
Pledged financial instruments, investment securities $ 0 $ 2,698
Pledged assets, mortgage assets held-for-investment and lending receivables 14,979 17,135
Pledged assets, restricted as collateral 35,798 46,204
Secured Debt 9,992 25,773
Short-term borrowings 2,136 5,531
Secured debt    
Pledged Assets related to secured borrowings [Line Items]    
Short-term borrowings 0 2,950
Consumer portfolio segment | Automotive loan    
Pledged Assets related to secured borrowings [Line Items]    
Pledged assets, finance receivables 9,953 13,481
Commercial portfolio segment | Automotive loan    
Pledged Assets related to secured borrowings [Line Items]    
Pledged assets, finance receivables 10,866 12,890
Ally Bank    
Pledged Assets related to secured borrowings [Line Items]    
Pledged financial instruments, investment securities 0 2,698
Pledged assets, mortgage assets held-for-investment and lending receivables 14,979 17,135
Pledged assets, restricted as collateral 35,355 44,257
Secured Debt 9,634 24,069
Ally Bank | Pledged assets for Federal Home Loan Bank    
Pledged Assets related to secured borrowings [Line Items]    
Pledged assets, restricted as collateral 20,000 24,800
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 portfolio segment | Automotive loan    
Pledged Assets related to secured borrowings [Line Items]    
Pledged assets, finance receivables 9,510 11,534
Ally Bank | Commercial portfolio segment | Automotive loan    
Pledged Assets related to secured borrowings [Line Items]    
Pledged assets, finance receivables $ 10,866 $ 12,890