Leasing (Ally as the Lessee) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022	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	$ 8	$ 9	$ 16	$ 19
Right-of-use asset obtained in exchange for operating lease liability			$ 1	$ 21
Operating lease, weighted-average remaining lease term	5 years		5 years		5 years
Operating lease, weighted average discount rate	2.61%		2.61%		2.57%
Minimum
Lessee, Lease, Description [Line Items]
Operating lease remaining lease term	2 months		2 months
Maximum
Lessee, Lease, Description [Line Items]
Operating lease remaining lease term	8 years		8 years