Leasing (Ally as the Lessee) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 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	$ 13
Right-of-use asset obtained in exchange for operating lease liability	$ 12	$ 337
Operating lease, weighted-average remaining lease term	5 years		6 years
Operating lease, weighted average discount rate	2.14%		1.96%
Undiscounted future lease payments	$ 13
Term of contract	10 years
Finance lease liabilities	$ 42		$ 0
Minimum
Lessee, Lease, Description [Line Items]
Operating lease remaining lease term	4 months
Option to terminate	5 years
Maximum
Lessee, Lease, Description [Line Items]
Operating lease remaining lease term	7 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