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	Jun. 30, 2018	Dec. 31, 2017
Available for sale securities:
Amortized Cost	$ 55,697	$ 50,029
Gross Unrealized Gains	84	105
Gross Unrealized Losses	259	139
Fair Value	55,522	49,995
Held to maturity securities:
Amortized Cost	136,792	120,926
Gross Unrealized Gains	209	497
Gross Unrealized Losses	3,009	1,050
Fair Value	133,992	120,373
U.S. agency mortgage-backed securities [Member]
Available for sale securities:
Amortized Cost	22,977	20,915
Gross Unrealized Gains	50	53
Gross Unrealized Losses	80	39
Fair Value	22,947	20,929
Held to maturity securities:
Amortized Cost	113,106	101,197
Gross Unrealized Gains	55	290
Gross Unrealized Losses	2,907	1,034
Fair Value	110,254	100,453
U.S. Treasury securities [Member]
Available for sale securities:
Amortized Cost	11,012	9,583
Gross Unrealized Gains	0	0
Gross Unrealized Losses	153	83
Fair Value	10,859	9,500
Held to maturity securities:
Amortized Cost	223	223
Gross Unrealized Gains	0	0
Gross Unrealized Losses	9	3
Fair Value	214	220
Asset-backed securities [Member]
Available for sale securities:
Amortized Cost	10,710	9,019
Gross Unrealized Gains	20	34
Gross Unrealized Losses	10	6
Fair Value	10,720	9,047
Held to maturity securities:
Amortized Cost	16,356	12,937
Gross Unrealized Gains	125	127
Gross Unrealized Losses	10	2
Fair Value	16,471	13,062
Corporate debt securities [Member]
Available for sale securities:
Amortized Cost	6,187	6,154
Gross Unrealized Gains	11	16
Gross Unrealized Losses	7	1
Fair Value	6,191	6,169
Held to maturity securities:
Amortized Cost	4,550	4,078
Gross Unrealized Gains	9	13
Gross Unrealized Losses	55	5
Fair Value	$ 4,504	$ 4,086
AFS and HTM securities percentage	35.00%	41.00%
Certificates of deposit [Member]
Available for sale securities:
Amortized Cost	$ 2,690	$ 2,040
Gross Unrealized Gains	3	2
Gross Unrealized Losses	1	1
Fair Value	2,692	2,041
Held to maturity securities:
Amortized Cost	200	200
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	200	200
U.S. agency notes [Member]
Available for sale securities:
Amortized Cost	1,530	1,914
Gross Unrealized Gains	0	0
Gross Unrealized Losses	6	8
Fair Value	1,524	1,906
U.S. state and municipal securities [Member]
Held to maturity securities:
Amortized Cost	1,242	1,247
Gross Unrealized Gains	18	57
Gross Unrealized Losses	3	0
Fair Value	1,257	1,304
Commercial paper [Member]
Available for sale securities:
Amortized Cost	505	313
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	505	313
Non-agency commercial mortgage-backed securities [Member]
Available for sale securities:
Amortized Cost	36	40
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	36	40
Held to maturity securities:
Amortized Cost	1,065	994
Gross Unrealized Gains	2	10
Gross Unrealized Losses	23	5
Fair Value	1,044	999
Foreign government agency securities [Member]
Available for sale securities:
Amortized Cost	50	51
Gross Unrealized Gains	0	0
Gross Unrealized Losses	2	1
Fair Value	48	50
Held to maturity securities:
Amortized Cost	50	50
Gross Unrealized Gains	0	0
Gross Unrealized Losses	2	1
Fair Value	$ 48	$ 49
Federal Family Education Loan Program (FFELP) Guaranteed Loans [Member]
Held to maturity securities:
Asset-backed securities percentage	36.00%	42.00%
Collateralized Credit Card Securities [Member]
Held to maturity securities:
Asset-backed securities percentage	46.00%	40.00%
Corporate Debt Securities, Information Technology [Member]
Held to maturity securities:
AFS and HTM securities percentage	21.00%	22.00%

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Concentration Risk, Available For Sale And Held To Maturity Securities

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

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

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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.

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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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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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