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Debt (Pledged Assets Related to Secured Borrowings and Repurchase Agreement) (Details) - USD ($)
Sep. 30, 2020
Dec. 31, 2019
Pledged Assets related to secured borrowings [Line Items]    
Pledged financial instruments, investment securities $ 0 $ 2,698,000,000
Pledged assets, mortgage assets held-for-investment and lending receivables 15,943,000,000 17,135,000,000
Pledged assets, restricted as collateral 37,921,000,000 46,204,000,000
Secured Debt 14,648,000,000 25,773,000,000
Short-term borrowings 3,032,000,000 5,531,000,000
Secured debt    
Pledged Assets related to secured borrowings [Line Items]    
Short-term borrowings 500,000,000 2,950,000,000
Consumer portfolio segment | Automotive loan    
Pledged Assets related to secured borrowings [Line Items]    
Pledged assets, finance receivables 11,167,000,000 13,481,000,000
Commercial portfolio segment | Automotive loan    
Pledged Assets related to secured borrowings [Line Items]    
Pledged assets, finance receivables 10,811,000,000 12,890,000,000
Ally Bank    
Pledged Assets related to secured borrowings [Line Items]    
Pledged financial instruments, investment securities 0 2,698,000,000
Pledged assets, mortgage assets held-for-investment and lending receivables 15,943,000,000 17,135,000,000
Pledged assets, restricted as collateral 37,273,000,000 44,257,000,000
Secured Debt 14,092,000,000 24,069,000,000
Ally Bank | Pledged assets for Federal Home Loan Bank    
Pledged Assets related to secured borrowings [Line Items]    
Pledged assets, restricted as collateral 20,900,000,000 24,800,000,000
Ally Bank | Pledged assets for Federal Reserve Bank    
Pledged Assets related to secured borrowings [Line Items]    
Pledged assets, restricted as collateral 2,400,000,000 2,400,000,000
Ally Bank | Consumer portfolio segment | Automotive loan    
Pledged Assets related to secured borrowings [Line Items]    
Pledged assets, finance receivables 10,519,000,000 11,534,000,000
Ally Bank | Commercial portfolio segment | Automotive loan    
Pledged Assets related to secured borrowings [Line Items]    
Pledged assets, finance receivables $ 10,811,000,000 $ 12,890,000,000