Long-Term Debt (Pledged Assets Related to Secured Borrowings And Repurchase Agreement) (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Pledged Assets related to secured borrowings [Line Items]
Pledged Financial Instruments, Not Separately Reported, Other Debt Securities Available-for-sale or Held-for-investment	$ 1,591	$ 786
Pledged Assets, Mortgage assets held for investment	9,746	7,541
Pledged Assets, Investment in operating leases	6,810	6,820
Pledged Assets, Other, Not Separately Reported on Statement of Financial Position	70,455	69,190
Secured Debt	49,681	47,867
Short-term borrowings	5,378	7,062
Ally Bank [Member]
Pledged Assets related to secured borrowings [Line Items]
Pledged Financial Instruments, Not Separately Reported, Other Debt Securities Available-for-sale or Held-for-investment	0	786
Pledged Assets, Mortgage assets held for investment	9,746	7,541
Pledged Assets, Investment in operating leases	3,947	4,672
Pledged Assets, Other, Not Separately Reported on Statement of Financial Position	40,865	44,345
Secured Debt	23,686	27,134
Pledged assets for Federal Home Loan Bank [Member] | Ally Bank [Member]
Pledged Assets related to secured borrowings [Line Items]
Pledged Assets, Other, Not Separately Reported on Statement of Financial Position	13,000	10,700
Pledged assets for Federal Reserve Bank [Member] | Ally Bank [Member]
Pledged Assets related to secured borrowings [Line Items]
Pledged Assets, Other, Not Separately Reported on Statement of Financial Position	3,000	3,200
Secured Debt [Member]
Pledged Assets related to secured borrowings [Line Items]
Short-term borrowings	1,920	3,724
ConsumerAutomobileMember [Member]
Pledged Assets related to secured borrowings [Line Items]
Pledged Assets, Not Separately Reported, Finance Receivables	34,186	33,438
ConsumerAutomobileMember [Member] | Ally Bank [Member]
Pledged Assets related to secured borrowings [Line Items]
Pledged Assets, Not Separately Reported, Finance Receivables	9,491	11,263
Commercial Automobile [Member]
Pledged Assets related to secured borrowings [Line Items]
Pledged Assets, Not Separately Reported, Finance Receivables	18,122	20,605
Commercial Automobile [Member] | Ally Bank [Member]
Pledged Assets related to secured borrowings [Line Items]
Pledged Assets, Not Separately Reported, Finance Receivables	$ 17,681	$ 20,083