Leasing (Ally as the Lessee) (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended
	Jun. 30, 2022	Mar. 31, 2023	Mar. 31, 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	$ 10
Right-of-use asset obtained in exchange for operating lease liability			$ 12
Operating lease, weighted-average remaining lease term		5 years		5 years
Operating lease, weighted average discount rate		2.59%		2.57%
Minimum
Lessee, Lease, Description [Line Items]
Operating lease remaining lease term		5 months
Maximum
Lessee, Lease, Description [Line Items]
Operating lease remaining lease term		8 years
Operations Center
Lessee, Lease, Description [Line Items]
Net property and equipment	$ 44