Finance Receivables and Loans, Net Finance Receivables and Loans, Net (Schedule of Average Balance and Interest Income of Impaired Finance Receivables) (Details) $ in Millions	3 Months Ended
Mar. 31, 2019 USD ($)
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	$ 1,033
Impaired Financing Receivable, Interest Income, Accrual Method	12
Consumer portfolio segment
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	728
Impaired Financing Receivable, Interest Income, Accrual Method	11
Commercial portfolio segment
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	305
Impaired Financing Receivable, Interest Income, Accrual Method	1
Automotive loan \| Consumer portfolio segment
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	499
Impaired Financing Receivable, Interest Income, Accrual Method	8
Automotive loan \| Commercial portfolio segment
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	170
Impaired Financing Receivable, Interest Income, Accrual Method	1
Other loan \| Commercial portfolio segment
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	130
Impaired Financing Receivable, Interest Income, Accrual Method	0
Real estate-backed loan \| Consumer portfolio segment
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	229
Impaired Financing Receivable, Interest Income, Accrual Method	3
Real estate-backed loan \| Commercial portfolio segment
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	5
Impaired Financing Receivable, Interest Income, Accrual Method	0
Mortgage Finance operations \| Consumer portfolio segment
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	15
Impaired Financing Receivable, Interest Income, Accrual Method	0
Mortgage - Legacy \| Consumer portfolio segment
Financing Receivable, Impaired [Line Items]
Impaired Financing Receivable, Average Recorded Investment	214
Impaired Financing Receivable, Interest Income, Accrual Method	$ 3

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.

No definition available.

+ Details

Name:	us-gaap_FinancingReceivableImpairedLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Average amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=118936363&loc=d3e5212-111524

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 17
-URI http://asc.fasb.org/extlink&oid=118936363&loc=d3e5258-111524

+ Details

Name:	us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of interest income recognized that the financing receivables (with related allowance for credit losses and without a related allowance for credit losses) were impaired.

+ References

+ Details

Name:	us-gaap_ImpairedFinancingReceivableInterestIncomeAccrualMethod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ConsumerPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_AutomobileLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=ally_OtherloanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_RealEstateLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=ally_MortgageFinanceMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=ally_MortgageLegacyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: