Leasing - Ally as the Lessee (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023	Dec. 31, 2023
Lessee, Lease, Description [Line Items]
Noncancelable lease term			367 days
Lease extension, maximum			48 months
Cash paid for amounts included in the measurement of lease liabilities	$ 9	$ 7	$ 26	$ 23
Right-of-use asset obtained in exchange for operating lease liability			$ 19	$ 10
Operating lease, weighted-average remaining lease term	4 years		4 years		4 years
Operating lease, weighted average discount rate	3.16%		3.16%		2.85%
Minimum
Lessee, Lease, Description [Line Items]
Operating lease remaining lease term	3 months		3 months
Maximum
Lessee, Lease, Description [Line Items]
Operating lease remaining lease term	6 years		6 years