Debt (Narrative - Trust Preferred Securities) (Details) - USD ($) $ / shares in Units, $ in Millions	Apr. 22, 2021	Mar. 31, 2021	Dec. 31, 2020
Narrative - Trust Preferred Securities [Line Items]
Trust preferred securities		$ 2,600	$ 2,600
Preferred stock, liquidation amount per share		$ 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		20
Redemption price, percentage of principal debt, plus accrued and unpaid interest		100.00%
Preferred Stock | Subsequent event
Narrative - Trust Preferred Securities [Line Items]
Preferred stock, liquidation amount per share	$ 1,000
Proceeds from Issuance of Preferred Stock and Preference Stock	$ 1,350
Variable Income Interest Rate
Narrative - Trust Preferred Securities [Line Items]
Trust preferred securities		$ 2,600	$ 2,600
Trust Preferred Securities Subject to Mandatory Redemption
Narrative - Trust Preferred Securities [Line Items]
Series 2 TRUPS included in Tier 1 capital		$ 2,500