Leasing - Ally as the Lessee (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2026	Mar. 31, 2025	Dec. 31, 2025
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	$ 9	$ 9
Right-of-use asset obtained in exchange for operating lease liability	$ 7	$ 5
Operating lease, weighted-average remaining lease term	5 years		5 years
Operating lease, weighted average discount rate	3.68%		3.66%
Minimum
Lessee, Lease, Description [Line Items]
Operating lease remaining lease term	1 month
Maximum
Lessee, Lease, Description [Line Items]
Operating lease remaining lease term	10 years

Cash Paid For Amounts Included In The Measurement Of Lease Liabilities

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- Definition

Lessee, Operating Lease, Lease Extension, Maximum

+ References

No definition available.

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- Definition

Lessee, Operating Lease, Noncancellable Lease Term

+ References

No definition available.

+ Details

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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- Definition

Remaining lease term of operating lease, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

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- Definition

Weighted average discount rate for operating lease calculated at point in time.

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- Definition

Weighted average remaining lease term for operating lease, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
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- Definition

Amount of increase in right-of-use asset obtained in exchange for operating lease liability.

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