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Debt (Pledged Assets Related to Secured Borrowings and Repurchase Agreement) (Details) - USD ($)
$ in Millions
Mar. 31, 2021
Dec. 31, 2020
Pledged Assets related to secured borrowings [Line Items]    
Pledged assets, mortgage assets held-for-investment and lending receivables $ 12,799 $ 14,979
Pledged assets, restricted as collateral 27,528 35,798
Secured Debt 8,701 9,992
Consumer portfolio segment | Automotive loan    
Pledged Assets related to secured borrowings [Line Items]    
Pledged assets, finance receivables 10,808 9,953
Commercial portfolio segment | Automotive loan    
Pledged Assets related to secured borrowings [Line Items]    
Pledged assets, finance receivables 3,921 10,866
Ally Bank    
Pledged Assets related to secured borrowings [Line Items]    
Pledged assets, mortgage assets held-for-investment and lending receivables 12,799 14,979
Pledged assets, restricted as collateral 27,182 35,355
Secured Debt 8,436 9,634
Ally Bank | Pledged assets for Federal Home Loan Bank    
Pledged Assets related to secured borrowings [Line Items]    
Pledged assets, restricted as collateral 12,800 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 portfolio segment | Automotive loan    
Pledged Assets related to secured borrowings [Line Items]    
Pledged assets, finance receivables 10,462 9,510
Ally Bank | Commercial portfolio segment | Automotive loan    
Pledged Assets related to secured borrowings [Line Items]    
Pledged assets, finance receivables $ 3,921 $ 10,866