Finance Receivables and Loans, Net (Allowance for Credit Losses on Financing Receivables) (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Financing Receivable, Allowance for Loan and Lease Losses [Roll Forward]
Financing Receivable, Allowance for Credit Losses	$ 1,278	$ 1,155	$ 1,276	$ 1,144
Allowance for Loan and Lease Losses, Write-offs	(306)	(296)	(683)	(646)
Allowance for Loan and Lease Loss, Recovery of Bad Debts	126	97	244	194
Allowance for Loan and Lease Losses Write-offs, Net	(180)	(199)	(439)	(452)
Provision for Loan Losses Expensed	158	269	419	540
Allowance for Loan and Lease Losses, Adjustments, Other	(1)		(1)	7
Financing Receivable, Allowance for Credit Losses	1,257	1,225	1,257	1,225
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	93	97	93	97
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	1,164	1,128	1,164	1,128
Loans and Leases Receivable, Gross	125,544	120,528	125,544	120,528	$ 122,893
Financing Receivable, Individually Evaluated for Impairment	906	773	906	773
Financing Receivable, Collectively Evaluated for Impairment	124,638	119,755	124,638	119,755
Consumer Portfolio Segment [Member] \| Automobile Loan [Member]
Financing Receivable, Allowance for Loan and Lease Losses [Roll Forward]
Financing Receivable, Allowance for Credit Losses	1,066	941	1,066	932
Allowance for Loan and Lease Losses, Write-offs	(296)	(290)	(661)	(631)
Allowance for Loan and Lease Loss, Recovery of Bad Debts	114	91	226	181
Allowance for Loan and Lease Losses Write-offs, Net	(182)	(199)	(435)	(450)
Provision for Loan Losses Expensed	168	260	421	527
Allowance for Loan and Lease Losses, Adjustments, Other	(1)		(1)	7
Financing Receivable, Allowance for Credit Losses	1,053	1,002	1,053	1,002
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	42	34	42	34
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	1,011	968	1,011	968
Loans and Leases Receivable, Gross	70,473	66,774	70,473	66,774	68,071
Financing Receivable, Individually Evaluated for Impairment	480	380	480	380
Financing Receivable, Collectively Evaluated for Impairment	69,993	66,394	69,993	66,394
Consumer Portfolio Segment [Member] \| Residential Mortgage [Member]
Financing Receivable, Allowance for Loan and Lease Losses [Roll Forward]
Financing Receivable, Allowance for Credit Losses	74	86	79	91
Allowance for Loan and Lease Losses, Write-offs	(8)	(6)	(20)	(15)
Allowance for Loan and Lease Loss, Recovery of Bad Debts	6	6	12	13
Allowance for Loan and Lease Losses Write-offs, Net	(2)	0	(8)	(2)
Provision for Loan Losses Expensed	(4)	(3)	(3)	(6)
Allowance for Loan and Lease Losses, Adjustments, Other	2		2	0
Financing Receivable, Allowance for Credit Losses	66	83	66	83
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	24	31	24	31
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	42	52	42	52
Loans and Leases Receivable, Gross	15,131	11,294	15,131	11,294	13,750
Financing Receivable, Individually Evaluated for Impairment	228	242	228	242
Financing Receivable, Collectively Evaluated for Impairment	14,903	11,052	14,903	11,052
Commercial Portfolio Segment [Member] \| Commercial Loan [Member]
Financing Receivable, Allowance for Loan and Lease Losses [Roll Forward]
Financing Receivable, Allowance for Credit Losses	138	128	131	121
Allowance for Loan and Lease Losses, Write-offs	(2)	0	(2)	0
Allowance for Loan and Lease Loss, Recovery of Bad Debts	6	0	6	0
Allowance for Loan and Lease Losses Write-offs, Net	4	0	4	0
Provision for Loan Losses Expensed	(6)	12	1	19
Allowance for Loan and Lease Losses, Adjustments, Other	(2)		(2)	0
Financing Receivable, Allowance for Credit Losses	138	140	138	140
Financing Receivable, Allowance for Credit Loss, Additional Information [Abstract]
Financing Receivable, Allowance for Credit Losses, Individually Evaluated for Impairment	27	32	27	32
Financing Receivable, Allowance for Credit Losses, Collectively Evaluated for Impairment	111	108	111	108
Loans and Leases Receivable, Gross	39,940	42,460	39,940	42,460	$ 41,072
Financing Receivable, Individually Evaluated for Impairment	198	151	198	151
Financing Receivable, Collectively Evaluated for Impairment	$ 39,742	$ 42,309	$ 39,742	$ 42,309

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

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- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5212-111524

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.7(d))
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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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No definition available.

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(3)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
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-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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

+ References

No definition available.

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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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=82891408&loc=d3e4428-111522

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Amount of expense related to loss from loan transactions.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 225
-Section S99
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