Debt (Committed Funding Facilities) (Details) - Committed funding facilities - Secured debt - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity	$ 2,500	$ 8,600
Long-term line of credit	450	6,665
Line of credit facility, remaining borrowing capacity	2,050	1,935
Revolving credit facility
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity	2,500
Secured funding facilities, revolving, one year or greater
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity	1,100
Ally Bank
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
Ally Financial Inc
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity	2,500	3,800
Long-term line of credit	450	3,165
Line of credit facility, remaining borrowing capacity	$ 2,050	$ 635