Long-Term Debt (Pledged Assets Related to Secured Borrowings And Repurchase Agreement) (Details) - USD ($) $ in Millions	Jun. 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	$ 2,062	$ 786
Pledged Assets, Mortgage assets held for investment	9,205	7,541
Pledged Assets, Investment in operating leases	7,648	6,820
Pledged Assets, Other, Not Separately Reported on Statement of Financial Position	72,081	69,190
Secured Debt	52,839	47,867
Short-term borrowings	10,013	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	9,205	7,541
Pledged Assets, Investment in operating leases	4,517	4,672
Pledged Assets, Other, Not Separately Reported on Statement of Financial Position	41,410	44,345
Secured Debt	25,693	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	12,400	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	3,100	3,200
Secured Debt [Member]
Pledged Assets related to secured borrowings [Line Items]
Short-term borrowings	6,554	3,724
Consumer Loans Auto Financing Receivable [Member]
Pledged Assets related to secured borrowings [Line Items]
Pledged Assets, Not Separately Reported, Finance Receivables	34,148	33,438
Consumer Loans Auto Financing Receivable [Member] | Ally Bank [Member]
Pledged Assets related to secured borrowings [Line Items]
Pledged Assets, Not Separately Reported, Finance Receivables	9,095	11,263
Commercial Automobile [Member]
Pledged Assets related to secured borrowings [Line Items]
Pledged Assets, Not Separately Reported, Finance Receivables	19,018	20,605
Commercial Automobile [Member] | Ally Bank [Member]
Pledged Assets related to secured borrowings [Line Items]
Pledged Assets, Not Separately Reported, Finance Receivables	$ 18,593	$ 20,083