Leasing (Ally as the Lessee) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 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	$ 7	$ 10	$ 23	$ 29
Right-of-use asset obtained in exchange for operating lease liability			$ 10	$ 36
Operating lease, weighted-average remaining lease term	4 years		4 years		5 years
Operating lease, weighted average discount rate	2.87%		2.87%		2.57%
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