Leasing (Ally as the Lessee) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	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	$ 10	$ 12	$ 39	$ 37
Right-of-use asset obtained in exchange for operating lease liability			$ 352	$ 58
Operating lease, weighted-average remaining lease term	7 years		7 years		7 years
Operating lease, weighted average discount rate	2.03%		2.03%		2.21%
Undiscounted future lease payments	$ 13		$ 13
Term of contract	10 years		10 years
Minimum
Lessee, Lease, Description [Line Items]
Operating lease remaining lease term	3 months		3 months
Option to terminate			5
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
Land and Building | Maximum
Lessee, Lease, Description [Line Items]
Option to extend			15 years