Leasing - Ally as the Lessee (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	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	$ 35	$ 33
Right-of-use asset obtained in exchange for operating lease liability	$ 34	$ 10
Operating lease, weighted-average remaining lease term	3 years	4 years
Operating lease, weighted average discount rate	3.32%	2.85%
Minimum
Lessee, Lease, Description [Line Items]
Operating lease remaining lease term	6 months
Maximum
Lessee, Lease, Description [Line Items]
Operating lease remaining lease term	6 years