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	Mar. 31, 2018	Dec. 31, 2017
Available for sale securities:
Amortized Cost	$ 51,969	$ 50,029
Gross Unrealized Gains	88	105
Gross Unrealized Losses	230	139
Fair Value	51,827	49,995
Held to maturity securities:
Amortized Cost	125,683	120,926
Gross Unrealized Gains	239	497
Gross Unrealized Losses	2,459	1,050
Fair Value	123,463	120,373
U.S. agency mortgage-backed securities [Member]
Available for sale securities:
Amortized Cost	21,077	20,915
Gross Unrealized Gains	49	53
Gross Unrealized Losses	65	39
Fair Value	21,061	20,929
Held to maturity securities:
Amortized Cost	103,967	101,197
Gross Unrealized Gains	82	290
Gross Unrealized Losses	2,377	1,034
Fair Value	101,672	100,453
U.S. Treasury securities [Member]
Available for sale securities:
Amortized Cost	10,964	9,583
Gross Unrealized Gains	0	0
Gross Unrealized Losses	137	83
Fair Value	10,827	9,500
Held to maturity securities:
Amortized Cost	223	223
Gross Unrealized Gains	0	0
Gross Unrealized Losses	8	3
Fair Value	215	220
Asset-backed securities [Member]
Available for sale securities:
Amortized Cost	9,622	9,019
Gross Unrealized Gains	25	34
Gross Unrealized Losses	11	6
Fair Value	9,636	9,047
Held to maturity securities:
Amortized Cost	14,625	12,937
Gross Unrealized Gains	126	127
Gross Unrealized Losses	7	2
Fair Value	14,744	13,062
Corporate debt securities [Member]
Available for sale securities:
Amortized Cost	6,546	6,154
Gross Unrealized Gains	12	16
Gross Unrealized Losses	6	1
Fair Value	6,552	6,169
Held to maturity securities:
Amortized Cost	4,340	4,078
Gross Unrealized Gains	8	13
Gross Unrealized Losses	44	5
Fair Value	$ 4,304	$ 4,086
AFS and HTM securities percentage	38.00%	41.00%
Certificates of deposit [Member]
Available for sale securities:
Amortized Cost	$ 1,790	$ 2,040
Gross Unrealized Gains	2	2
Gross Unrealized Losses	1	1
Fair Value	1,791	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	1,565	1,914
Gross Unrealized Gains	0	0
Gross Unrealized Losses	8	8
Fair Value	1,557	1,906
Commercial paper [Member]
Available for sale securities:
Amortized Cost	315	313
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	315	313
U.S. state and municipal securities [Member]
Held to maturity securities:
Amortized Cost	1,245	1,247
Gross Unrealized Gains	20	57
Gross Unrealized Losses	3	0
Fair Value	1,262	1,304
Non-agency commercial mortgage-backed securities [Member]
Available for sale securities:
Amortized Cost	40	40
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	40	40
Held to maturity securities:
Amortized Cost	1,033	994
Gross Unrealized Gains	3	10
Gross Unrealized Losses	19	5
Fair Value	1,017	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	1	1
Fair Value	$ 49	$ 49
Federal Family Education Loan Program (FFELP) Guaranteed Loans [Member]
Held to maturity securities:
Asset-backed securities percentage	40.00%	42.00%
Collateralized Credit Card Securities [Member]
Held to maturity securities:
Asset-backed securities percentage	42.00%	40.00%
Corporate Debt Securities, Information Technology [Member]
Held to maturity securities:
AFS and HTM securities percentage	22.00%	22.00%

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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.

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-Publisher FASB
-Name Accounting Standards Codification
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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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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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