Finance Receivables and Loans, Net - Schedule of Allowance for Credit Losses on Financing Receivables (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Financing Receivable, Excluding Accrued Interest, Allowance for Credit Loss [Roll Forward]
Allowance, beginning balance	$ 3,587	$ 3,711
Charge-offs	(2,948)	(2,720)
Recoveries	914	833
Net charge-offs	(2,034)	(1,887)
Write-downs from transfers to held-for-sale	(5)	(215)
Provision for credit losses	2,166	1,979
Other	0	(1)
Allowance, ending balance	3,714	3,587
Disposal Group, Held-for-Sale, Not Discontinued Operations | Ally Lending
Financing Receivable, Excluding Accrued Interest, Allowance for Credit Loss [Roll Forward]
Unfunded lending commitments		5
Unfunded Loan Commitment
Financing Receivable, Excluding Accrued Interest, Allowance for Credit Loss [Roll Forward]
Provision for credit losses		(11)
Consumer
Financing Receivable, Excluding Accrued Interest, Allowance for Credit Loss [Roll Forward]
Charge-offs	(2,945)	(2,590)
Consumer | Automotive
Financing Receivable, Excluding Accrued Interest, Allowance for Credit Loss [Roll Forward]
Allowance, beginning balance	3,083	3,020
Charge-offs	(2,681)	(2,284)
Recoveries	871	793
Net charge-offs	(1,810)	(1,491)
Write-downs from transfers to held-for-sale	(5)	(41)
Provision for credit losses	1,902	1,595
Other	0	0
Allowance, ending balance	3,170	3,083
Consumer | Consumer mortgage
Financing Receivable, Excluding Accrued Interest, Allowance for Credit Loss [Roll Forward]
Allowance, beginning balance	21	27
Charge-offs	(2)	(3)
Recoveries	5	9
Net charge-offs	3	6
Write-downs from transfers to held-for-sale	0	0
Provision for credit losses	(7)	(11)
Other	2	(1)
Allowance, ending balance	19	21
Consumer | Consumer other
Financing Receivable, Excluding Accrued Interest, Allowance for Credit Loss [Roll Forward]
Allowance, beginning balance	293	426
Charge-offs	(262)	(303)
Recoveries	30	25
Net charge-offs	(232)	(278)
Write-downs from transfers to held-for-sale	0	(174)
Provision for credit losses	259	319
Other	(1)	0
Allowance, ending balance	319	293
Fair value, option, carrying amount, financing receivable, no allowance			$ 3
Commercial
Financing Receivable, Excluding Accrued Interest, Allowance for Credit Loss [Roll Forward]
Allowance, beginning balance	190	238
Charge-offs	(3)	(130)
Recoveries	8	6
Net charge-offs	5	(124)
Write-downs from transfers to held-for-sale	0	0
Provision for credit losses	12	76
Other	(1)	0
Allowance, ending balance	206	190
Commercial | Automotive
Financing Receivable, Excluding Accrued Interest, Allowance for Credit Loss [Roll Forward]
Charge-offs	$ (3)	(24)
Commercial | Consumer other
Financing Receivable, Excluding Accrued Interest, Allowance for Credit Loss [Roll Forward]
Charge-offs		$ (106)