Finance Receivables and Loans, Net (Allowance for Credit Losses on Financing Receivables) (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Financing Receivable, Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan losses	$ 1,257	$ 1,225	$ 1,276	$ 1,144
Allowance for loan and lease losses, charge-offs	(353)	(344)	(1,036)	(990)
Allowance for loan and lease losses, recoveries	118	91	362	285
Allowance for loan and lease losses, net charge-offs	(235)	(253)	(674)	(705)
Provision for loan losses	233	314	652	854
Allowance for loan and lease losses, other	7	0	6	7
Allowance for loan losses	1,248	1,286	1,248	1,286
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Allowance for loan losses, individually evaluated for impairment	102	86	102	86
Allowance for loan losses, collectively evaluated for impairment	1,146	1,200	1,146	1,200
Finance receivables and loans, net	126,605	118,871	126,605	118,871	$ 122,893
Finance receivables and loans, individually evaluated for impairment	898	786	898	786
Finance receivables and loans, collectively evaluated for impairment	125,707	118,085	125,707	118,085
Consumer portfolio segment \| Automobile loan
Financing Receivable, Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan losses	1,053	1,002	1,066	932
Allowance for loan and lease losses, charge-offs	(343)	(327)	(1,004)	(958)
Allowance for loan and lease losses, recoveries	110	85	336	266
Allowance for loan and lease losses, net charge-offs	(233)	(242)	(668)	(692)
Provision for loan losses	229	314	650	841
Allowance for loan and lease losses, other	6	0	5	7
Allowance for loan losses	1,043	1,074	1,043	1,074
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Allowance for loan losses, individually evaluated for impairment	43	35	43	35
Allowance for loan losses, collectively evaluated for impairment	1,000	1,039	1,000	1,039
Finance receivables and loans, net	69,995	67,077	69,995	67,077	68,071
Finance receivables and loans, individually evaluated for impairment	483	403	483	403
Finance receivables and loans, collectively evaluated for impairment	69,512	66,674	69,512	66,674
Consumer portfolio segment \| Residential mortgage
Financing Receivable, Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan losses	66	83	79	91
Allowance for loan and lease losses, charge-offs	(7)	(7)	(27)	(22)
Allowance for loan and lease losses, recoveries	8	6	20	19
Allowance for loan and lease losses, net charge-offs	1	(1)	(7)	(3)
Provision for loan losses	(4)	0	(7)	(6)
Allowance for loan and lease losses, other	(1)	1	1	1
Allowance for loan losses	64	81	64	81
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Allowance for loan losses, individually evaluated for impairment	24	30	24	30
Allowance for loan losses, collectively evaluated for impairment	40	51	40	51
Finance receivables and loans, net	16,506	12,015	16,506	12,015	13,750
Finance receivables and loans, individually evaluated for impairment	231	237	231	237
Finance receivables and loans, collectively evaluated for impairment	16,275	11,778	16,275	11,778
Commercial portfolio segment \| Commercial loan
Financing Receivable, Allowance for Loan and Lease Losses [Roll Forward]
Allowance for loan losses	138	140	131	121
Allowance for loan and lease losses, charge-offs	(3)	(10)	(5)	(10)
Allowance for loan and lease losses, recoveries	0	0	6	0
Allowance for loan and lease losses, net charge-offs	(3)	(10)	1	(10)
Provision for loan losses	8	0	9	19
Allowance for loan and lease losses, other	2	(1)	0	(1)
Allowance for loan losses	141	131	141	131
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Allowance for loan losses, individually evaluated for impairment	35	21	35	21
Allowance for loan losses, collectively evaluated for impairment	106	110	106	110
Finance receivables and loans, net	40,104	39,779	40,104	39,779	$ 41,072
Finance receivables and loans, individually evaluated for impairment	184	146	184	146
Finance receivables and loans, collectively evaluated for impairment	$ 39,920	$ 39,633	$ 39,920	$ 39,633

Amount of recovery of loans and lease receivables which had previously been fully or partially written-off as bad debts.

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Amount of other addition (reduction) in allowance reserve for credit loss.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Amount of loans and leases that have been written off from both loan receivables and allowance reserve for credit loss.

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Amount, after recoveries, of loans and leases that have been written-off.

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A valuation allowance for financing receivables that are expected to be uncollectible.

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The valuation allowance for financing receivables that are expected to be uncollectible that were collectively evaluated for impairment.

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Amount of valuation allowance for financing receivables that are expected to be uncollectible that were individually evaluated for impairment.

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The balance of financing receivables that were collectively evaluated for impairment.

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The balance of financing receivables that were individually evaluated for impairment.

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Amount before allowance of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements.

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Amount of expense related to loss from loan transactions.

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