Debt (Narrative - Trust Preferred Securities) (Details) - USD ($) $ / shares in Units, $ in Billions	Sep. 30, 2020	Dec. 31, 2019
Narrative - Trust Preferred Securities [Line Items]
Trust preferred securities	$ 2.6	$ 2.6
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%
Variable Income Interest Rate
Narrative - Trust Preferred Securities [Line Items]
Trust preferred securities	$ 2.6	$ 2.6
Trust Preferred Securities Subject to Mandatory Redemption
Narrative - Trust Preferred Securities [Line Items]
Series 2 TRUPS included in Tier 1 capital	$ 2.5