Leasing (Ally as the Lessee) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
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	$ 51	$ 49
Right-of-use asset obtained in exchange for operating lease liability	$ 361	$ 93
Operating lease, weighted-average remaining lease term	6 years	7 years
Operating lease, weighted average discount rate	1.96%	2.21%
Undiscounted future lease payments	$ 13
Term of contract	10 years
Lease income	$ 1
Minimum
Lessee, Lease, Description [Line Items]
Operating lease remaining lease term	7 months
Option to terminate	5 years
Maximum
Lessee, Lease, Description [Line Items]
Operating lease remaining lease term	10 years
Option to terminate	6 years
Land and Building | Minimum
Lessee, Lease, Description [Line Items]
Option to extend	1 year
Land and Building | Maximum
Lessee, Lease, Description [Line Items]
Option to extend	15 years