Leasing - Ally as the Lessee (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 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	$ 8	$ 17	$ 16
Right-of-use asset obtained in exchange for operating lease liability			$ 16	$ 1
Operating lease, weighted-average remaining lease term	4 years		4 years		4 years
Operating lease, weighted average discount rate	3.13%		3.13%		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	7 years		7 years