Short-Term Borrowings (Schedule of Short-term Debt) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Short-term Debt [Line Items]
Demand Notes	$ 3,338ally_DemandNotes	$ 3,225ally_DemandNotes
Federal Home Loan Bank, Advances, Short-term	2,950	3,570
Securities Sold under Agreements to Repurchase, Gross	774	1,500
Other Short-term Borrowings	0us-gaap_OtherShortTermBorrowings	250us-gaap_OtherShortTermBorrowings
Short-term borrowings	7,062us-gaap_ShortTermBorrowings	8,545us-gaap_ShortTermBorrowings
Short-term Debt, Weighted Average Interest Rate	0.80%	0.80%
Unsecured Debt [Member]
Short-term Debt [Line Items]
Demand Notes	3,338	3,225
Federal Home Loan Bank, Advances, Short-term	0	0
Securities Sold under Agreements to Repurchase, Gross	0	0
Other Short-term Borrowings	0	0
Short-term borrowings	3,338	3,225
Secured Debt [Member]
Short-term Debt [Line Items]
Demand Notes	0	0
Federal Home Loan Bank, Advances, Short-term	2,950	3,570
Securities Sold under Agreements to Repurchase, Gross	774	1,500
Other Short-term Borrowings	0	250
Short-term borrowings	$ 3,724	$ 5,320

Demand Notes

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Amount of advances from Federal Home Loan Bank (FHLBank) maturing in the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Sum of the carrying amounts at the balance sheet date of short-term borrowings not otherwise specified in the taxonomy having initial terms less than one year or the normal operating cycle, if longer.

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Amount, before effects of master netting arrangements, of funds outstanding borrowed in the form of a security repurchase agreement between the entity and another party for the sale and repurchase of identical or substantially the same securities at a date certain for a specified price. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

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Reflects the total carrying amount as of the balance sheet date of debt having initial terms less than one year or the normal operating cycle, if longer.

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Weighted average interest rate of short-term debt outstanding.

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