Label	Element	Value
AOCI Attributable to Parent [Member]
Stockholders' Equity Attributable to Parent	us-gaap_StockholdersEquity	$ 123,000,000
Stockholders' Equity Attributable to Parent	us-gaap_StockholdersEquity	(531,000,000)
Treasury Stock [Member]
Stockholders' Equity Attributable to Parent	us-gaap_StockholdersEquity	(2,049,000,000)
Stockholders' Equity Attributable to Parent	us-gaap_StockholdersEquity	(3,088,000,000)
Common Stock Including Additional Paid in Capital [Member]
Stockholders' Equity Attributable to Parent	us-gaap_StockholdersEquity	21,345,000,000
Stockholders' Equity Attributable to Parent	us-gaap_StockholdersEquity	21,438,000,000
Mortgage - Legacy [Member] | Consumer Portfolio Segment [Member]
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	us-gaap_FinancingReceivableOriginatedInFiscalYearBeforeLatestFiscalYear	0
Financing Receivable, Originated Four Years before Latest Fiscal Year	us-gaap_FinancingReceivableOriginatedFourYearsBeforeLatestFiscalYear	0
Financing Receivable, Originated Two Years before Latest Fiscal Year	us-gaap_FinancingReceivableOriginatedTwoYearsBeforeLatestFiscalYear	0
Financing Receivable, Originated Three Years before Latest Fiscal Year	us-gaap_FinancingReceivableOriginatedThreeYearsBeforeLatestFiscalYear	0
Converted to term loans	ally_Convertedtotermloans	123,000,000
Financing Receivable, Revolving	us-gaap_FinancingReceivableRevolving	351,000,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	us-gaap_FinancingReceivableOriginatedFiveOrMoreYearsBeforeLatestFiscalYear	510,000,000
Financing Receivable, Originated in Current Fiscal Year	us-gaap_FinancingReceivableOriginatedInCurrentFiscalYear	0
Mortgage - Legacy [Member] | Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Consumer Portfolio Segment [Member]
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	us-gaap_FinancingReceivableOriginatedInFiscalYearBeforeLatestFiscalYear	0
Financing Receivable, Originated Four Years before Latest Fiscal Year	us-gaap_FinancingReceivableOriginatedFourYearsBeforeLatestFiscalYear	0
Financing Receivable, Originated Two Years before Latest Fiscal Year	us-gaap_FinancingReceivableOriginatedTwoYearsBeforeLatestFiscalYear	0
Financing Receivable, Originated Three Years before Latest Fiscal Year	us-gaap_FinancingReceivableOriginatedThreeYearsBeforeLatestFiscalYear	0
Converted to term loans	ally_Convertedtotermloans	1,000,000
Financing Receivable, Revolving	us-gaap_FinancingReceivableRevolving	5,000,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	us-gaap_FinancingReceivableOriginatedFiveOrMoreYearsBeforeLatestFiscalYear	17,000,000
Financing Receivable, Originated in Current Fiscal Year	us-gaap_FinancingReceivableOriginatedInCurrentFiscalYear	0
Mortgage - Legacy [Member] | Financial Asset, 60 to 89 Days Past Due [Member] | Consumer Portfolio Segment [Member]
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	us-gaap_FinancingReceivableOriginatedInFiscalYearBeforeLatestFiscalYear	0
Financing Receivable, Originated Four Years before Latest Fiscal Year	us-gaap_FinancingReceivableOriginatedFourYearsBeforeLatestFiscalYear	0
Financing Receivable, Originated Two Years before Latest Fiscal Year	us-gaap_FinancingReceivableOriginatedTwoYearsBeforeLatestFiscalYear	0
Financing Receivable, Originated Three Years before Latest Fiscal Year	us-gaap_FinancingReceivableOriginatedThreeYearsBeforeLatestFiscalYear	0
Converted to term loans	ally_Convertedtotermloans	0
Financing Receivable, Revolving	us-gaap_FinancingReceivableRevolving	1,000,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	us-gaap_FinancingReceivableOriginatedFiveOrMoreYearsBeforeLatestFiscalYear	3,000,000
Financing Receivable, Originated in Current Fiscal Year	us-gaap_FinancingReceivableOriginatedInCurrentFiscalYear	0
Mortgage - Legacy [Member] | Financial Asset, 30 to 59 Days Past Due [Member] | Consumer Portfolio Segment [Member]
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	us-gaap_FinancingReceivableOriginatedInFiscalYearBeforeLatestFiscalYear	0
Financing Receivable, Originated Four Years before Latest Fiscal Year	us-gaap_FinancingReceivableOriginatedFourYearsBeforeLatestFiscalYear	0
Financing Receivable, Originated Two Years before Latest Fiscal Year	us-gaap_FinancingReceivableOriginatedTwoYearsBeforeLatestFiscalYear	0
Financing Receivable, Originated Three Years before Latest Fiscal Year	us-gaap_FinancingReceivableOriginatedThreeYearsBeforeLatestFiscalYear	0
Converted to term loans	ally_Convertedtotermloans	2,000,000
Financing Receivable, Revolving	us-gaap_FinancingReceivableRevolving	2,000,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	us-gaap_FinancingReceivableOriginatedFiveOrMoreYearsBeforeLatestFiscalYear	8,000,000
Financing Receivable, Originated in Current Fiscal Year	us-gaap_FinancingReceivableOriginatedInCurrentFiscalYear	0
Mortgage - Legacy [Member] | Current period [Member] | Consumer Portfolio Segment [Member]
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	us-gaap_FinancingReceivableOriginatedInFiscalYearBeforeLatestFiscalYear	0
Financing Receivable, Originated Four Years before Latest Fiscal Year	us-gaap_FinancingReceivableOriginatedFourYearsBeforeLatestFiscalYear	0
Financing Receivable, Originated Two Years before Latest Fiscal Year	us-gaap_FinancingReceivableOriginatedTwoYearsBeforeLatestFiscalYear	0
Financing Receivable, Originated Three Years before Latest Fiscal Year	us-gaap_FinancingReceivableOriginatedThreeYearsBeforeLatestFiscalYear	0
Converted to term loans	ally_Convertedtotermloans	120,000,000
Financing Receivable, Revolving	us-gaap_FinancingReceivableRevolving	343,000,000
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	us-gaap_FinancingReceivableOriginatedFiveOrMoreYearsBeforeLatestFiscalYear	482,000,000
Financing Receivable, Originated in Current Fiscal Year	us-gaap_FinancingReceivableOriginatedInCurrentFiscalYear	0
Loans and Leases Receivable, Gross	us-gaap_LoansAndLeasesReceivableGrossCarryingAmount	945,000,000
Mortgage Finance [Member] | Consumer Portfolio Segment [Member]
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	us-gaap_FinancingReceivableOriginatedInFiscalYearBeforeLatestFiscalYear	3,014,000,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	us-gaap_FinancingReceivableOriginatedFourYearsBeforeLatestFiscalYear	1,391,000,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	us-gaap_FinancingReceivableOriginatedTwoYearsBeforeLatestFiscalYear	2,397,000,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	us-gaap_FinancingReceivableOriginatedThreeYearsBeforeLatestFiscalYear	3,167,000,000
Converted to term loans	ally_Convertedtotermloans	1,000,000
Financing Receivable, Revolving	us-gaap_FinancingReceivableRevolving	0
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	us-gaap_FinancingReceivableOriginatedFiveOrMoreYearsBeforeLatestFiscalYear	4,403,000,000
Financing Receivable, Originated in Current Fiscal Year	us-gaap_FinancingReceivableOriginatedInCurrentFiscalYear	2,056,000,000
Mortgage Finance [Member] | Financial Asset, Equal to or Greater than 90 Days Past Due [Member] | Consumer Portfolio Segment [Member]
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	us-gaap_FinancingReceivableOriginatedInFiscalYearBeforeLatestFiscalYear	0
Financing Receivable, Originated Four Years before Latest Fiscal Year	us-gaap_FinancingReceivableOriginatedFourYearsBeforeLatestFiscalYear	1,000,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	us-gaap_FinancingReceivableOriginatedTwoYearsBeforeLatestFiscalYear	2,000,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	us-gaap_FinancingReceivableOriginatedThreeYearsBeforeLatestFiscalYear	1,000,000
Converted to term loans	ally_Convertedtotermloans	0
Financing Receivable, Revolving	us-gaap_FinancingReceivableRevolving	0
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	us-gaap_FinancingReceivableOriginatedFiveOrMoreYearsBeforeLatestFiscalYear	6,000,000
Financing Receivable, Originated in Current Fiscal Year	us-gaap_FinancingReceivableOriginatedInCurrentFiscalYear	0
Mortgage Finance [Member] | Financial Asset, 60 to 89 Days Past Due [Member] | Consumer Portfolio Segment [Member]
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	us-gaap_FinancingReceivableOriginatedInFiscalYearBeforeLatestFiscalYear	1,000,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	us-gaap_FinancingReceivableOriginatedFourYearsBeforeLatestFiscalYear	4,000,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	us-gaap_FinancingReceivableOriginatedTwoYearsBeforeLatestFiscalYear	3,000,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	us-gaap_FinancingReceivableOriginatedThreeYearsBeforeLatestFiscalYear	1,000,000
Converted to term loans	ally_Convertedtotermloans	0
Financing Receivable, Revolving	us-gaap_FinancingReceivableRevolving	0
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	us-gaap_FinancingReceivableOriginatedFiveOrMoreYearsBeforeLatestFiscalYear	3,000,000
Financing Receivable, Originated in Current Fiscal Year	us-gaap_FinancingReceivableOriginatedInCurrentFiscalYear	0
Mortgage Finance [Member] | Financial Asset, 30 to 59 Days Past Due [Member] | Consumer Portfolio Segment [Member]
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	us-gaap_FinancingReceivableOriginatedInFiscalYearBeforeLatestFiscalYear	7,000,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	us-gaap_FinancingReceivableOriginatedFourYearsBeforeLatestFiscalYear	11,000,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	us-gaap_FinancingReceivableOriginatedTwoYearsBeforeLatestFiscalYear	12,000,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	us-gaap_FinancingReceivableOriginatedThreeYearsBeforeLatestFiscalYear	14,000,000
Converted to term loans	ally_Convertedtotermloans	0
Financing Receivable, Revolving	us-gaap_FinancingReceivableRevolving	0
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	us-gaap_FinancingReceivableOriginatedFiveOrMoreYearsBeforeLatestFiscalYear	30,000,000
Financing Receivable, Originated in Current Fiscal Year	us-gaap_FinancingReceivableOriginatedInCurrentFiscalYear	9,000,000
Mortgage Finance [Member] | Current period [Member] | Consumer Portfolio Segment [Member]
Financing Receivable, Originated in Fiscal Year before Latest Fiscal Year	us-gaap_FinancingReceivableOriginatedInFiscalYearBeforeLatestFiscalYear	3,006,000,000
Financing Receivable, Originated Four Years before Latest Fiscal Year	us-gaap_FinancingReceivableOriginatedFourYearsBeforeLatestFiscalYear	1,375,000,000
Financing Receivable, Originated Two Years before Latest Fiscal Year	us-gaap_FinancingReceivableOriginatedTwoYearsBeforeLatestFiscalYear	2,380,000,000
Financing Receivable, Originated Three Years before Latest Fiscal Year	us-gaap_FinancingReceivableOriginatedThreeYearsBeforeLatestFiscalYear	3,151,000,000
Converted to term loans	ally_Convertedtotermloans	1,000,000
Financing Receivable, Revolving	us-gaap_FinancingReceivableRevolving	0
Financing Receivable, Originated Five or More Years before Latest Fiscal Year	us-gaap_FinancingReceivableOriginatedFiveOrMoreYearsBeforeLatestFiscalYear	4,364,000,000
Financing Receivable, Originated in Current Fiscal Year	us-gaap_FinancingReceivableOriginatedInCurrentFiscalYear	2,047,000,000
Loans and Leases Receivable, Gross	us-gaap_LoansAndLeasesReceivableGrossCarryingAmount	16,324,000,000
Accounting Standards Update 2017-08 [Member]
Stockholders' Equity Attributable to Parent	us-gaap_StockholdersEquity	(2,000,000)
Accounting Standards Update 2017-08 [Member] | AOCI Attributable to Parent [Member]
Stockholders' Equity Attributable to Parent	us-gaap_StockholdersEquity	8,000,000
Accounting Standards Update 2017-08 [Member] | Retained Earnings [Member]
Stockholders' Equity Attributable to Parent	us-gaap_StockholdersEquity	$ (10,000,000)