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Debt (Narrative - Trust Preferred Securities) (Details)
$ / shares in Units, $ in Millions
12 Months Ended
Oct. 15, 2021
USD ($)
shares
Jul. 02, 2021
USD ($)
shares
Jun. 02, 2021
USD ($)
May 24, 2021
USD ($)
shares
Apr. 22, 2021
USD ($)
Dec. 31, 2021
USD ($)
quarter
$ / shares
Dec. 31, 2020
USD ($)
Dec. 31, 2019
USD ($)
Jun. 30, 2021
$ / shares
Apr. 30, 2021
$ / shares
Debt Instrument [Line Items]                    
Liquidation preference (in dollars per share) | $ / shares           $ 25        
Distribution payable in addition to annual rate equal to three-month London interbank offer rate, percentage           5.785%        
Period of consecutive quarters for which Ally has right to defer interest payments, maximum | quarter           20        
Redemption price, percentage of principal debt, plus accrued and unpaid interest           100.00%        
Preferred stock issuance           $ 2,324 $ 0 $ 0    
Trust preferred securities redemption           $ 2,710 0 $ 0    
Series B Preferred Stock                    
Debt Instrument [Line Items]                    
Liquidation preference (in dollars per share) | $ / shares           $ 1,000       $ 1,000
Preferred stock issuance         $ 1,350          
Trust Preferred Securities Subject to Mandatory Redemption                    
Debt Instrument [Line Items]                    
Trust preferred securities redemption $ 191 $ 1,040   $ 1,400            
Debt redeemed during period, number of shares | shares 7,650,000 41,600,000   56,000,000            
Series C Preferred Stock                    
Debt Instrument [Line Items]                    
Liquidation preference (in dollars per share) | $ / shares           $ 1,000     $ 1,000  
Preferred stock issuance     $ 1,000              
Variable Income Interest Rate                    
Debt Instrument [Line Items]                    
Trust preferred securities             $ 2,600