Equity in Net Assets of Non-consolidated Affiliate (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Noncontrolling Interest [Line Items]
Net income	$ 45,000,000	$ 41,000,000	$ 97,000,000	$ 88,000,000
SAIC-GMAC [Member]
Noncontrolling Interest [Line Items]
Finance charge income	315,000,000	256,000,000	622,000,000	514,000,000
Provision for loan losses	6,000,000	4,000,000	9,000,000	(11,000,000)
Interest expense	132,000,000	82,000,000	256,000,000	158,000,000
Income before income taxes	171,000,000	155,000,000	369,000,000	333,000,000
Net income	$ 129,000,000	$ 116,000,000	$ 277,000,000	250,000,000
Ownership interest (percent)	35.00%		35.00%
Cash dividends received			$ 0	$ 0
Undistributed earnings	$ 412,000,000		$ 412,000,000		$ 315,000,000