Finance Receivables and Loans, Net (Past Due Financing Receivables and Loans) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	$ 2,967	$ 2,759
Financing Receivable, Recorded Investment, Current	119,926	116,185
Loans and Leases Receivable, Gross	122,893	118,944
Consumer Portfolio Segment [Member] | Consumer Loan [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	2,959	2,749
Financing Receivable, Recorded Investment, Current	78,862	74,094
Loans and Leases Receivable, Gross	81,821	76,843
Consumer Portfolio Segment [Member] | Automobile Loan [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	2,740	2,580
Financing Receivable, Recorded Investment, Current	65,331	63,213
Loans and Leases Receivable, Gross	68,071	65,793
Consumer Portfolio Segment [Member] | Residential Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	219	169
Financing Receivable, Recorded Investment, Current	13,531	10,881
Loans and Leases Receivable, Gross	13,750	11,050
Consumer Portfolio Segment [Member] | Mortgage Finance [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	89	49
Financing Receivable, Recorded Investment, Current	11,568	8,245
Loans and Leases Receivable, Gross	11,657	8,294
Consumer Portfolio Segment [Member] | Mortgage - Legacy [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	130	120
Financing Receivable, Recorded Investment, Current	1,963	2,636
Loans and Leases Receivable, Gross	2,093	2,756
Commercial Portfolio Segment [Member] | Commercial Loan [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	8	10
Financing Receivable, Recorded Investment, Current	41,064	42,091
Loans and Leases Receivable, Gross	41,072	42,101
Commercial Portfolio Segment [Member] | Commercial And Industrial Automobile [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	8	10
Financing Receivable, Recorded Investment, Current	33,017	35,031
Loans and Leases Receivable, Gross	33,025	35,041
Commercial Portfolio Segment [Member] | Commercial And Industrial Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	0	0
Financing Receivable, Recorded Investment, Current	3,887	3,248
Loans and Leases Receivable, Gross	3,887	3,248
Commercial Portfolio Segment [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	0	0
Financing Receivable, Recorded Investment, Current	4,160	3,812
Loans and Leases Receivable, Gross	4,160	3,812
Financing Receivables, 30 to 59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	2,102	1,937
Financing Receivables, 30 to 59 Days Past Due [Member] | Consumer Portfolio Segment [Member] | Consumer Loan [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	2,097	1,934
Financing Receivables, 30 to 59 Days Past Due [Member] | Consumer Portfolio Segment [Member] | Automobile Loan [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	1,994	1,850
Financing Receivables, 30 to 59 Days Past Due [Member] | Consumer Portfolio Segment [Member] | Residential Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	103	84
Financing Receivables, 30 to 59 Days Past Due [Member] | Consumer Portfolio Segment [Member] | Mortgage Finance [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	60	39
Financing Receivables, 30 to 59 Days Past Due [Member] | Consumer Portfolio Segment [Member] | Mortgage - Legacy [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	43	45
Financing Receivables, 30 to 59 Days Past Due [Member] | Commercial Portfolio Segment [Member] | Commercial Loan [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	5	3
Financing Receivables, 30 to 59 Days Past Due [Member] | Commercial Portfolio Segment [Member] | Commercial And Industrial Automobile [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	5	3
Financing Receivables, 30 to 59 Days Past Due [Member] | Commercial Portfolio Segment [Member] | Commercial And Industrial Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	0	0
Financing Receivables, 30 to 59 Days Past Due [Member] | Commercial Portfolio Segment [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	0	0
Financing Receivables, 60 to 89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	514	452
Financing Receivables, 60 to 89 Days Past Due [Member] | Consumer Portfolio Segment [Member] | Consumer Loan [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	514	452
Financing Receivables, 60 to 89 Days Past Due [Member] | Consumer Portfolio Segment [Member] | Automobile Loan [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	478	428
Financing Receivables, 60 to 89 Days Past Due [Member] | Consumer Portfolio Segment [Member] | Residential Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	36	24
Financing Receivables, 60 to 89 Days Past Due [Member] | Consumer Portfolio Segment [Member] | Mortgage Finance [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	11	6
Financing Receivables, 60 to 89 Days Past Due [Member] | Consumer Portfolio Segment [Member] | Mortgage - Legacy [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	25	18
Financing Receivables, 60 to 89 Days Past Due [Member] | Commercial Portfolio Segment [Member] | Commercial Loan [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	0	0
Financing Receivables, 60 to 89 Days Past Due [Member] | Commercial Portfolio Segment [Member] | Commercial And Industrial Automobile [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	0	0
Financing Receivables, 60 to 89 Days Past Due [Member] | Commercial Portfolio Segment [Member] | Commercial And Industrial Other [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	0	0
Financing Receivables, 60 to 89 Days Past Due [Member] | Commercial Portfolio Segment [Member] | Commercial Real Estate [Member]
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 [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	351	370
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Consumer Portfolio Segment [Member] | Consumer Loan [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	348	363
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Consumer Portfolio Segment [Member] | Automobile Loan [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	268	302
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Consumer Portfolio Segment [Member] | Residential Mortgage [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	80	61
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Consumer Portfolio Segment [Member] | Mortgage Finance [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	18	4
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Consumer Portfolio Segment [Member] | Mortgage - Legacy [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	62	57
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Commercial Portfolio Segment [Member] | Commercial Loan [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	3	7
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Commercial Portfolio Segment [Member] | Commercial And Industrial Automobile [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	3	7
Financing Receivables, Equal to Greater than 90 Days Past Due [Member] | Commercial Portfolio Segment [Member] | Commercial And Industrial Other [Member]
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 [Member] | Commercial Portfolio Segment [Member] | Commercial Real Estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Financing Receivable, Recorded Investment, Past Due	$ 0	$ 0