Debt (Committed Funding Facilities) (Details) - Committed funding facilities - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity	$ 3,300
Secured debt
Debt Instrument [Line Items]
Long-term line of credit	1,615	$ 6,665
Line of credit facility, remaining borrowing capacity	1,635	1,935
Line of credit facility, maximum borrowing capacity	3,250	8,600
Secured funding facilities, revolving, one year or greater
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity	2,000
Ally Financial Inc | Secured debt
Debt Instrument [Line Items]
Long-term line of credit	1,365	3,165
Line of credit facility, remaining borrowing capacity	1,635	635
Line of credit facility, maximum borrowing capacity	3,000	3,800
Ally Bank | Secured debt
Debt Instrument [Line Items]
Long-term line of credit	250	3,500
Line of credit facility, remaining borrowing capacity	0	1,300
Line of credit facility, maximum borrowing capacity	$ 250	$ 4,800