Debt (Committed Funding Facilities) (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Revolving credit facility
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity	$ 1,600
Revolving secured funding facilities 1 year or greater
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity	550
Committed funding facilities | Secured debt
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity	1,550	$ 2,500
Long-term line of credit	0	450
Line of credit facility, remaining borrowing capacity	1,550	2,050
Committed funding facilities | Ally Financial Inc | Secured debt
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity		2,500
Long-term line of credit	0	450
Line of credit facility, remaining borrowing capacity	1,550	$ 2,050
Committed funding facilities | Ally Financial Inc | Revolving credit facility | Secured debt
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity	$ 1,550