Debt (Committed Funding Facilities) (Details) - Committed funding facilities - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity	$ 2,700
Secured debt
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity	2,650	$ 8,600
Long-term line of credit	700	6,665
Line of credit facility, remaining borrowing capacity	1,950	1,935
Secured funding facilities, revolving, one year or greater
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity	1,700
Ally Financial Inc | Secured debt
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity	2,650	3,800
Long-term line of credit	700	3,165
Line of credit facility, remaining borrowing capacity	1,950	635
Ally Bank | Secured debt
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity	0	4,800
Long-term line of credit	0	3,500
Line of credit facility, remaining borrowing capacity	$ 0	$ 1,300