Debt (Narrative - Trust Preferred Securities) (Details) $ / shares in Units, $ in Billions	Mar. 31, 2019 USD ($) $ / shares	Dec. 31, 2018 USD ($)
Narrative - Trust Preferred Securities [Line Items]
Trust preferred securities	$ 2.6	$ 2.6
Trust preferred securities, fixed rate, floating rate, percentage	8.125%
Preferred stock, liquidation amount 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	20
Redemption price, percentage of principal debt, plus accrued and unpaid interest	100.00%
Fixed income interest rate
Narrative - Trust Preferred Securities [Line Items]
Trust preferred securities	$ 2.6	$ 2.6