Investment Securities (Amortized Cost, Gross Unrealized Gains and Losses, and Fair Value of Securities Available for Sale and Held to Maturity Securities) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Available for sale securities:
Amortized Cost	$ 50,029	$ 77,627
Gross Unrealized Gains	105	176
Gross Unrealized Losses	139	438
Fair Value	49,995	77,365
Held to maturity securities:
Amortized Cost	120,926	75,203
Gross Unrealized Gains	497	336
Gross Unrealized Losses	1,050	1,095
Fair Value	120,373	74,444
U.S. agency mortgage-backed securities [Member]
Available for sale securities:
Amortized Cost	20,915	33,167
Gross Unrealized Gains	53	120
Gross Unrealized Losses	39	92
Fair Value	20,929	33,195
Held to maturity securities:
Amortized Cost	101,197	72,439
Gross Unrealized Gains	290	324
Gross Unrealized Losses	1,034	1,086
Fair Value	100,453	71,677
U.S. Treasury securities [Member]
Available for sale securities:
Amortized Cost	9,583	8,679
Gross Unrealized Gains	0	3
Gross Unrealized Losses	83	59
Fair Value	9,500	8,623
Held to maturity securities:
Amortized Cost	223	223
Gross Unrealized Gains	0	0
Gross Unrealized Losses	3	4
Fair Value	220	219
Asset-backed securities [Member]
Available for sale securities:
Amortized Cost	9,019	20,520
Gross Unrealized Gains	34	29
Gross Unrealized Losses	6	214
Fair Value	9,047	20,335
Held to maturity securities:
Amortized Cost	12,937	941
Gross Unrealized Gains	127	0
Gross Unrealized Losses	2	0
Fair Value	13,062	941
Corporate debt securities [Member]
Available for sale securities:
Amortized Cost	6,154	9,850
Gross Unrealized Gains	16	20
Gross Unrealized Losses	1	18
Fair Value	6,169	9,852
Held to maturity securities:
Amortized Cost	4,078	436
Gross Unrealized Gains	13	0
Gross Unrealized Losses	5	0
Fair Value	$ 4,086	$ 436
Available for sale and held to maturity securities	41.00%	49.00%
Certificates of deposit [Member]
Available for sale securities:
Amortized Cost	$ 2,040	$ 2,070
Gross Unrealized Gains	2	2
Gross Unrealized Losses	1	1
Fair Value	2,041	2,071
Held to maturity securities:
Amortized Cost	200
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Fair Value	200
U.S. agency notes [Member]
Available for sale securities:
Amortized Cost	1,914	1,915
Gross Unrealized Gains	0	0
Gross Unrealized Losses	8	8
Fair Value	1,906	1,907
Commercial paper [Member]
Available for sale securities:
Amortized Cost	313	214
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	313	214
Held to maturity securities:
Amortized Cost		99
Gross Unrealized Gains		0
Gross Unrealized Losses		0
Fair Value		99
U.S. state and municipal securities [Member]
Available for sale securities:
Amortized Cost		1,167
Gross Unrealized Gains		2
Gross Unrealized Losses		46
Fair Value		1,123
Held to maturity securities:
Amortized Cost	1,247	68
Gross Unrealized Gains	57	1
Gross Unrealized Losses	0	1
Fair Value	1,304	68
Non-agency commercial mortgage-backed securities [Member]
Available for sale securities:
Amortized Cost	40	45
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	40	45
Held to maturity securities:
Amortized Cost	994	997
Gross Unrealized Gains	10	11
Gross Unrealized Losses	5	4
Fair Value	999	$ 1,004
Foreign government agency securities [Member]
Available for sale securities:
Amortized Cost	51
Gross Unrealized Gains	0
Gross Unrealized Losses	1
Fair Value	50
Held to maturity securities:
Amortized Cost	50
Gross Unrealized Gains	0
Gross Unrealized Losses	1
Fair Value	$ 49
Federal Family Education Loan Program Asset-Backed Securities [Member]
Held to maturity securities:
Asset-backed securities percent	42.00%	47.00%
Collateralized Credit Card Securities [Member]
Held to maturity securities:
Asset-backed securities percent	40.00%	36.00%
Corporate Debt Securities, Information Technology [Member]
Held to maturity securities:
Available for sale and held to maturity securities	22.00%	19.00%

Concentration Risk, Asset-Backed Securities

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Concentration Risk, Available For Sale And Held To Maturity Securities

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This item represents the cost of debt securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
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-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.2)
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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in equity securities classified as available-for-sale.

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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Amount of debt securities categorized neither as held-to-maturity nor trading.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558

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-Topic 320
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-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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Amount after other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-Section 50
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-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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Amount of accumulated unrealized gain on investments in debt securities classified as held-to-maturity.

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Amount of accumulated unrealized loss on investments in debt securities classified as held-to-maturity.

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Fair value of investments in debt securities classified as held-to-maturity.

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