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	Sep. 30, 2017	Dec. 31, 2016
Available for sale securities:
Amortized Cost	$ 48,023	$ 77,627
Gross Unrealized Gains	124	176
Gross Unrealized Losses	85	438
Fair Value	48,062	77,365
Held to maturity securities:
Amortized Cost	114,376	75,203
Gross Unrealized Gains	681	336
Gross Unrealized Losses	725	1,095
Fair Value	114,332	74,444
U.S. agency mortgage-backed securities [Member]
Available for sale securities:
Amortized Cost	19,717	33,167
Gross Unrealized Gains	54	120
Gross Unrealized Losses	25	92
Fair Value	19,746	33,195
Held to maturity securities:
Amortized Cost	96,045	72,439
Gross Unrealized Gains	492	324
Gross Unrealized Losses	721	1,086
Fair Value	95,816	71,677
Asset-backed securities [Member]
Available for sale securities:
Amortized Cost	9,960	20,520
Gross Unrealized Gains	40	29
Gross Unrealized Losses	4	214
Fair Value	9,996	20,335
Held to maturity securities:
Amortized Cost	12,237	941
Gross Unrealized Gains	100	0
Gross Unrealized Losses	1	0
Fair Value	12,336	941
Corporate debt securities [Member]
Available for sale securities:
Amortized Cost	6,449	9,850
Gross Unrealized Gains	20	20
Gross Unrealized Losses	1	18
Fair Value	6,468	9,852
Held to maturity securities:
Amortized Cost	3,377	436
Gross Unrealized Gains	26	0
Gross Unrealized Losses	0	0
Fair Value	3,403	436
U.S. Treasury securities [Member]
Available for sale securities:
Amortized Cost	7,741	8,679
Gross Unrealized Gains	7	3
Gross Unrealized Losses	49	59
Fair Value	7,699	8,623
Held to maturity securities:
Amortized Cost	223	223
Gross Unrealized Gains	0	0
Gross Unrealized Losses	1	4
Fair Value	222	219
Certificates of deposit [Member]
Available for sale securities:
Amortized Cost	1,840	2,070
Gross Unrealized Gains	3	2
Gross Unrealized Losses	0	1
Fair Value	1,843	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	1,913	1,915
Gross Unrealized Gains	0	0
Gross Unrealized Losses	6	8
Fair Value	1,907	1,907
Commercial paper [Member]
Available for sale securities:
Amortized Cost	312	214
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	312	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	1,249	68
Gross Unrealized Gains	50	1
Gross Unrealized Losses	0	1
Fair Value	1,299	68
Non-agency commercial mortgage-backed securities [Member]
Available for sale securities:
Amortized Cost	41	45
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	41	45
Held to maturity securities:
Amortized Cost	995	997
Gross Unrealized Gains	13	11
Gross Unrealized Losses	2	4
Fair Value	1,006	$ 1,004
Foreign government agency securities [Member]
Available for sale securities:
Amortized Cost	50
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Fair Value	50
Held to maturity securities:
Amortized Cost	50
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Fair Value	$ 50

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.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
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-Publisher FASB
-Name Accounting Standards Codification
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-Section S99
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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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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
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Amount of accumulated unrealized gain 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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