Leasing - Ally as the Lessee (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024	Dec. 31, 2024
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	$ 9	$ 18	$ 17
Right-of-use asset obtained in exchange for operating lease liability			$ 6	$ 16
Operating lease, weighted-average remaining lease term	3 years		3 years		3 years
Operating lease, weighted average discount rate	3.38%		3.38%		3.32%
Minimum
Lessee, Lease, Description [Line Items]
Operating lease remaining lease term	1 month		1 month
Maximum
Lessee, Lease, Description [Line Items]
Operating lease remaining lease term	6 years		6 years