Finance Receivables and Loans, Net (Allowance for Credit Losses on Financing Receivables) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Financing Receivable, Allowance for Loan and Lease Losses [Roll Forward]
Financing Receivable, Allowance for Credit Losses				$ 1,144				$ 1,054	$ 1,144	$ 1,054
Allowance for Loan and Lease Losses, Write-offs									(1,392)	(1,142)
Allowance for Loan and Lease Loss, Recovery of Bad Debts									382	341
Allowance for Loan and Lease Losses Write-offs, Net									(1,010)	(801)
Provision for Loan Losses Expensed	$ 294	$ 314	$ 269	271	$ 267	$ 258	$ 172	220	1,148	917	$ 707
Allowance for Loan and Lease Losses, Adjustments, Other									6	26
Financing Receivable, Allowance for Credit Losses	1,276				1,144				1,276	1,144	1,054
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	77				85				77	85
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	1,199				1,059				1,199	1,059
Loans and Leases Receivable, Gross	122,893				118,944				122,893	118,944
Financing Receivable, Individually Evaluated for Impairment	733				739				733	739
Financing Receivable, Collectively Evaluated for Impairment	122,160				118,205				122,160	118,205
Consumer Portfolio Segment [Member] | Automobile Loan [Member]
Financing Receivable, Allowance for Loan and Lease Losses [Roll Forward]
Financing Receivable, Allowance for Credit Losses				932				834	932	834
Allowance for Loan and Lease Losses, Write-offs									(1,344)	(1,102)
Allowance for Loan and Lease Loss, Recovery of Bad Debts									358	307
Allowance for Loan and Lease Losses Write-offs, Net									(986)	(795)
Provision for Loan Losses Expensed									1,127	919
Allowance for Loan and Lease Losses, Adjustments, Other									7	26
Financing Receivable, Allowance for Credit Losses	1,066				932				1,066	932	834
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	36				28				36	28
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	1,030				904				1,030	904
Loans and Leases Receivable, Gross	68,071				65,793				68,071	65,793
Financing Receivable, Individually Evaluated for Impairment	430				370				430	370
Financing Receivable, Collectively Evaluated for Impairment	67,641				65,423				67,641	65,423
Consumer Portfolio Segment [Member] | Residential Mortgage [Member]
Financing Receivable, Allowance for Loan and Lease Losses [Roll Forward]
Financing Receivable, Allowance for Credit Losses				91				114	91	114
Allowance for Loan and Lease Losses, Write-offs									(30)	(39)
Allowance for Loan and Lease Loss, Recovery of Bad Debts									24	32
Allowance for Loan and Lease Losses Write-offs, Net									(6)	(7)
Provision for Loan Losses Expensed									(7)	(16)
Allowance for Loan and Lease Losses, Adjustments, Other									(1)	0
Financing Receivable, Allowance for Credit Losses	79				91				79	91	114
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	27				34				27	34
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	52				57				52	57
Loans and Leases Receivable, Gross	13,750				11,050				13,750	11,050
Financing Receivable, Individually Evaluated for Impairment	231				247				231	247
Financing Receivable, Collectively Evaluated for Impairment	13,519				10,803				13,519	10,803
Commercial Portfolio Segment [Member] | Commercial Loan [Member]
Financing Receivable, Allowance for Loan and Lease Losses [Roll Forward]
Financing Receivable, Allowance for Credit Losses				$ 121				$ 106	121	106
Allowance for Loan and Lease Losses, Write-offs									(18)	(1)
Allowance for Loan and Lease Loss, Recovery of Bad Debts									0	2
Allowance for Loan and Lease Losses Write-offs, Net									(18)	1
Provision for Loan Losses Expensed									28	14
Allowance for Loan and Lease Losses, Adjustments, Other									0	0
Financing Receivable, Allowance for Credit Losses	131				121				131	121	$ 106
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	14				23				14	23
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	117				98				117	98
Loans and Leases Receivable, Gross	41,072				42,101				41,072	42,101
Financing Receivable, Individually Evaluated for Impairment	72				122				72	122
Financing Receivable, Collectively Evaluated for Impairment	$ 41,000				$ 41,979				$ 41,000	$ 41,979