Acquisition of Ally Financial International Operations - Narrative (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended		12 Months Ended
	Jan. 02, 2015	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2013	Jan. 03, 2015
Business Acquisition [Line Items]
Equity income		$ 29	$ 0	$ 85	$ 0
SAIC-GMAC [Member]
Business Acquisition [Line Items]
Equity interest (percent)	40.00%						35.00%
Aggregate purchase price	$ 1,000
Equity interest (percent)	5.00%
Proceeds from sale	$ 125
Difference between carrying amount and underlying equity	$ 371
Equity income		$ 29	$ 183	$ 85	$ 561
Ally Financial International Operations [Member]
Business Acquisition [Line Items]
Aggregate consideration						$ 3,300
Debt assumed						$ 1,400