Leasing (Ally as the Lessee) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
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	$ 33	$ 38
Right-of-use asset obtained in exchange for operating lease liability	$ 10	$ 41
Operating lease, weighted-average remaining lease term	4 years	5 years
Operating lease, weighted average discount rate	2.85%	2.57%
Minimum
Lessee, Lease, Description [Line Items]
Operating lease remaining lease term	2 months
Maximum
Lessee, Lease, Description [Line Items]
Operating lease remaining lease term	7 years