Finance Receivables and Loans, Net (Past Due Financing Receivables and Loans) (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivable, recorded investment, past due	$ 2,742	$ 2,967
Financing receivable, recorded investment, current	123,863	119,926
Total finance receivables and loans	126,605	122,893	$ 118,871
Consumer portfolio segment | Consumer loan
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivable, recorded investment, past due	2,708	2,959
Financing receivable, recorded investment, current	83,793	78,862
Total finance receivables and loans	86,501	81,821
Consumer portfolio segment | Automobile loan
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivable, recorded investment, past due	2,535	2,740
Financing receivable, recorded investment, current	67,460	65,331
Total finance receivables and loans	69,995	68,071	67,077
Consumer portfolio segment | Residential mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivable, recorded investment, past due	173	219
Financing receivable, recorded investment, current	16,333	13,531
Total finance receivables and loans	16,506	13,750	12,015
Consumer portfolio segment | Mortgage Finance
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivable, recorded investment, past due	72	89
Financing receivable, recorded investment, current	14,768	11,568
Total finance receivables and loans	14,840	11,657
Consumer portfolio segment | Mortgage - Legacy
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivable, recorded investment, past due	101	130
Financing receivable, recorded investment, current	1,565	1,963
Total finance receivables and loans	1,666	2,093
Commercial portfolio segment | Commercial loan
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivable, recorded investment, past due	34	8
Financing receivable, recorded investment, current	40,070	41,064
Total finance receivables and loans	40,104	41,072	$ 39,779
Commercial portfolio segment | Commercial and industrial automotive
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivable, recorded investment, past due	15	8
Financing receivable, recorded investment, current	31,409	33,017
Total finance receivables and loans	31,424	33,025
Commercial portfolio segment | Commercial and industrial other
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivable, recorded investment, past due	19	0
Financing receivable, recorded investment, current	4,113	3,887
Total finance receivables and loans	4,132	3,887
Commercial portfolio segment | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivable, recorded investment, past due	0	0
Financing receivable, recorded investment, current	4,548	4,160
Total finance receivables and loans	4,548	4,160
Financing receivables, 30 to 59 days past due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivable, recorded investment, past due	1,927	2,102
Financing receivables, 30 to 59 days past due | Consumer portfolio segment | Consumer loan
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivable, recorded investment, past due	1,923	2,097
Financing receivables, 30 to 59 days past due | Consumer portfolio segment | Automobile loan
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivable, recorded investment, past due	1,831	1,994
Financing receivables, 30 to 59 days past due | Consumer portfolio segment | Residential mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivable, recorded investment, past due	92	103
Financing receivables, 30 to 59 days past due | Consumer portfolio segment | Mortgage Finance
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivable, recorded investment, past due	56	60
Financing receivables, 30 to 59 days past due | Consumer portfolio segment | Mortgage - Legacy
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivable, recorded investment, past due	36	43
Financing receivables, 30 to 59 days past due | Commercial portfolio segment | Commercial loan
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivable, recorded investment, past due	4	5
Financing receivables, 30 to 59 days past due | Commercial portfolio segment | Commercial and industrial automotive
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivable, recorded investment, past due	0	5
Financing receivables, 30 to 59 days past due | Commercial portfolio segment | Commercial and industrial other
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivable, recorded investment, past due	4	0
Financing receivables, 30 to 59 days past due | Commercial portfolio segment | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivable, recorded investment, past due	0	0
Financing receivables, 60 to 89 days past due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivable, recorded investment, past due	462	514
Financing receivables, 60 to 89 days past due | Consumer portfolio segment | Consumer loan
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivable, recorded investment, past due	462	514
Financing receivables, 60 to 89 days past due | Consumer portfolio segment | Automobile loan
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivable, recorded investment, past due	442	478
Financing receivables, 60 to 89 days past due | Consumer portfolio segment | Residential mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivable, recorded investment, past due	20	36
Financing receivables, 60 to 89 days past due | Consumer portfolio segment | Mortgage Finance
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivable, recorded investment, past due	6	11
Financing receivables, 60 to 89 days past due | Consumer portfolio segment | Mortgage - Legacy
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivable, recorded investment, past due	14	25
Financing receivables, 60 to 89 days past due | Commercial portfolio segment | Commercial loan
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivable, recorded investment, past due	0	0
Financing receivables, 60 to 89 days past due | Commercial portfolio segment | Commercial and industrial automotive
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivable, recorded investment, past due	0	0
Financing receivables, 60 to 89 days past due | Commercial portfolio segment | Commercial and industrial other
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivable, recorded investment, past due	0	0
Financing receivables, 60 to 89 days past due | Commercial portfolio segment | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivable, recorded investment, past due	0	0
Financing receivables, equal to greater than 90 days past due
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivable, recorded investment, past due	353	351
Financing receivables, equal to greater than 90 days past due | Consumer portfolio segment | Consumer loan
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivable, recorded investment, past due	323	348
Financing receivables, equal to greater than 90 days past due | Consumer portfolio segment | Automobile loan
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivable, recorded investment, past due	262	268
Financing receivables, equal to greater than 90 days past due | Consumer portfolio segment | Residential mortgage
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivable, recorded investment, past due	61	80
Financing receivables, equal to greater than 90 days past due | Consumer portfolio segment | Mortgage Finance
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivable, recorded investment, past due	10	18
Financing receivables, equal to greater than 90 days past due | Consumer portfolio segment | Mortgage - Legacy
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivable, recorded investment, past due	51	62
Financing receivables, equal to greater than 90 days past due | Commercial portfolio segment | Commercial loan
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivable, recorded investment, past due	30	3
Financing receivables, equal to greater than 90 days past due | Commercial portfolio segment | Commercial and industrial automotive
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivable, recorded investment, past due	15	3
Financing receivables, equal to greater than 90 days past due | Commercial portfolio segment | Commercial and industrial other
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivable, recorded investment, past due	15	0
Financing receivables, equal to greater than 90 days past due | Commercial portfolio segment | Commercial real estate
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing receivable, recorded investment, past due	$ 0	$ 0