Offsetting Assets and Liabilities (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Offsetting Assets and Liabilities [Line Items]
Derivative asset, gross amount of recognized assets	$ 22	$ 41
Derivative liability, gross amounts of recognized liabilities	21	37
Noncash Derivative Collateral Pledged to Ally	3	3
Noncash Repurchase Collateral pledged to Ally		4
Customer securities, entity has right to sell or repledge, fair value	3	7
Derivative assets in net asset positions
Offsetting Assets and Liabilities [Line Items]
Derivative asset, gross amount of recognized assets	20	41
Derivative assets and liabilities, gross amounts offset	0	0
Derivative asset, fair value, net amounts offset against collateral	20	41
Derivative asset, gross amounts not offset	1	0
Derivative, Collateral, Obligation to Return Cash	(1)	(4)
Derivative assets, net	18	37
Derivative assets with no offsetting arrangements [Member] [Domain]
Offsetting Assets and Liabilities [Line Items]
Derivative asset, gross amount of recognized assets	2
Derivative assets and liabilities, gross amounts offset	0
Derivative asset, fair value, net amounts offset against collateral	2
Derivative asset, gross amounts not offset	0
Derivative, Collateral, Obligation to Return Cash	0
Derivative assets, net	2
Derivative assets
Offsetting Assets and Liabilities [Line Items]
Derivative asset, gross amount of recognized assets	22	41
Derivative assets and liabilities, gross amounts offset	0	0
Derivative asset, fair value, net amounts offset against collateral	22	41
Derivative asset, gross amounts not offset	1	0
Derivative, Collateral, Obligation to Return Cash	(1)	(4)
Derivative assets, net	20	37
Derivative liabilities in net liability positions
Offsetting Assets and Liabilities [Line Items]
Derivative assets and liabilities, gross amounts offset	0	0
Derivative liabilities, collateral	(1)	0
Derivative liability, gross amounts of recognized liabilities	20	37
Derivative liability, fair value, net amounts offset against collateral	20	37
Derivative liability, gross amounts not offset	0	0
Derivative liabilities, net	19	37
Derivative liabilities in net asset positions [Member]
Offsetting Assets and Liabilities [Line Items]
Derivative assets and liabilities, gross amounts offset	0
Derivative liabilities, collateral	0
Derivative liability, gross amounts of recognized liabilities	1
Derivative liability, fair value, net amounts offset against collateral	1
Derivative liability, gross amounts not offset	(1)
Derivative liabilities, net	0
Total derivative liabilities excluding repurchase agreements [Member]
Offsetting Assets and Liabilities [Line Items]
Derivative assets and liabilities, gross amounts offset	0
Derivative liabilities, collateral	(1)
Derivative liability, gross amounts of recognized liabilities	21
Derivative liability, fair value, net amounts offset against collateral	21
Derivative liability, gross amounts not offset	(1)
Derivative liabilities, net	19
Securities sold under agreements to repurchase
Offsetting Assets and Liabilities [Line Items]
Derivative assets and liabilities, gross amounts offset	0	0
Derivative liabilities, collateral	(854)	(685)
Derivative liability, gross amounts of recognized liabilities	854	685
Derivative liability, fair value, net amounts offset against collateral	854	685
Derivative liability, gross amounts not offset	0	0
Derivative liabilities, net	0	0
Derivative liabilities
Offsetting Assets and Liabilities [Line Items]
Derivative assets and liabilities, gross amounts offset	0	0
Derivative liabilities, collateral	(855)	(685)
Derivative liability, gross amounts of recognized liabilities	875	722
Derivative liability, fair value, net amounts offset against collateral	875	722
Derivative liability, gross amounts not offset	(1)	0
Derivative liabilities, net	$ 19	$ 37