Leasing (Ally as the Lessee) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Mar. 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	$ 16	$ 12	$ 29	$ 25
Right-of-use asset obtained in exchange for operating lease liability			$ 333	$ 43
Operating lease, weighted-average remaining lease term	7 years		7 years			7 years
Operating lease, weighted average discount rate	2.15%		2.15%			2.21%
Finance lease liabilities	$ 326		$ 326			$ 0
Undiscounted future lease payments					$ 13
Term of contract					10 years
Minimum
Lessee, Lease, Description [Line Items]
Operating lease remaining lease term	3 months		3 months
Option to terminate			5 years
Maximum
Lessee, Lease, Description [Line Items]
Operating lease remaining lease term	11 years		11 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