Leasing (Ally as the Lessee) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021	Dec. 31, 2021
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	$ 10	$ 10	$ 29	$ 39
Right-of-use asset obtained in exchange for operating lease liability			$ 36	$ 352
Operating lease, weighted-average remaining lease term	5 years		5 years		6 years
Operating lease, weighted average discount rate	2.38%		2.38%		1.96%
Undiscounted future lease payments	$ 13		$ 13
Term of contract	10 years		10 years
ROU asset acquired	$ 44		$ 44
Minimum
Lessee, Lease, Description [Line Items]
Operating lease remaining lease term	2 months		2 months
Option to terminate			5 years
Maximum
Lessee, Lease, Description [Line Items]
Operating lease remaining lease term	8 years		8 years
Option to terminate			6 years
Land and Building | Minimum
Lessee, Lease, Description [Line Items]
Option to extend	1 year		1 year
Land and Building | Maximum
Lessee, Lease, Description [Line Items]
Option to extend	15 years		15 years