MSR (Sales of Receivables and Servicing Rights - Additional Information) (Detail) - USD ($) $ in Millions		3 Months Ended	12 Months Ended
		Mar. 31, 2015	Dec. 31, 2017	Dec. 31, 2016		Dec. 31, 2015
Servicing Assets at Fair Value
Fair value of automobile loan securitization servicing asset			$ 858	$ 744
Residential mortgage loan sales	[1]		6,369	6,927	[2]	$ 5,078
Sevicing of residential mortgage loans for other investors			$ 60,000	53,600
Amortization				$ (131)		$ (140)
Residential Mortgage Loans | Troubled Debt Restructuring On Nonaccrual Status
Servicing Assets at Fair Value
Gain on sale of HFS loans		$ 37
Residential mortgage loan sales		$ 568

[1] Represents the unpaid principal balance at the time of the sale .

[2] Excludes $ 568 of HFS residential mortgage loans previously modified in a TDR that were sold d uring the first quarter of 2015.