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, 2018	Dec. 31, 2017
Available for sale securities:
Amortized Cost	$ 57,776	$ 50,029
Gross Unrealized Gains	80	105
Gross Unrealized Losses	298	139
Fair Value	57,558	49,995
Held to maturity securities:
Amortized Cost	138,952	120,926
Gross Unrealized Gains	164	497
Gross Unrealized Losses	3,595	1,050
Fair Value	135,521	120,373
U.S. agency mortgage-backed securities [Member]
Available for sale securities:
Amortized Cost	24,010	20,915
Gross Unrealized Gains	47	53
Gross Unrealized Losses	102	39
Fair Value	23,955	20,929
Held to maturity securities:
Amortized Cost	113,453	101,197
Gross Unrealized Gains	26	290
Gross Unrealized Losses	3,483	1,034
Fair Value	109,996	100,453
U.S. Treasury securities [Member]
Available for sale securities:
Amortized Cost	12,412	9,583
Gross Unrealized Gains	0	0
Gross Unrealized Losses	169	83
Fair Value	12,243	9,500
Held to maturity securities:
Amortized Cost	223	223
Gross Unrealized Gains	0	0
Gross Unrealized Losses	11	3
Fair Value	212	220
Asset-backed securities [Member]
Available for sale securities:
Amortized Cost	9,378	9,019
Gross Unrealized Gains	19	34
Gross Unrealized Losses	10	6
Fair Value	9,387	9,047
Held to maturity securities:
Amortized Cost	17,964	12,937
Gross Unrealized Gains	122	127
Gross Unrealized Losses	12	2
Fair Value	18,074	13,062
Corporate debt securities [Member]
Available for sale securities:
Amortized Cost	6,921	6,154
Gross Unrealized Gains	10	16
Gross Unrealized Losses	9	1
Fair Value	6,922	6,169
Held to maturity securities:
Amortized Cost	4,578	4,078
Gross Unrealized Gains	8	13
Gross Unrealized Losses	55	5
Fair Value	4,531	4,086
Certificates of deposit [Member]
Available for sale securities:
Amortized Cost	2,765	2,040
Gross Unrealized Gains	4	2
Gross Unrealized Losses	0	1
Fair Value	2,769	2,041
Held to maturity securities:
Amortized Cost	200	200
Gross Unrealized Gains	1	0
Gross Unrealized Losses	0	0
Fair Value	201	200
U.S. agency notes [Member]
Available for sale securities:
Amortized Cost	1,688	1,914
Gross Unrealized Gains	0	0
Gross Unrealized Losses	6	8
Fair Value	1,682	1,906
U.S. state and municipal securities [Member]
Held to maturity securities:
Amortized Cost	1,330	1,247
Gross Unrealized Gains	6	57
Gross Unrealized Losses	7	0
Fair Value	1,329	1,304
Commercial paper [Member]
Available for sale securities:
Amortized Cost	518	313
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	518	313
Available-for-sale securities, current	2,000
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
Non-agency commercial mortgage-backed securities [Member]
Available for sale securities:
Amortized Cost	34	40
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	34	40
Held to maturity securities:
Amortized Cost	1,149	994
Gross Unrealized Gains	1	10
Gross Unrealized Losses	25	5
Fair Value	$ 1,125	$ 999
Federal Family Education Loan Program (FFELP) Guaranteed Loans [Member]
Held to maturity securities:
Asset-backed securities percentage	37.00%	42.00%
Collateralized Credit Card Securities [Member]
Held to maturity securities:
Asset-backed securities percentage	43.00%	40.00%
Other [Member]
Held to maturity securities:
Amortized Cost	$ 5
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Fair Value	$ 5
Corporate debt securities issued by financial services industry [Member]
Held to maturity securities:
AFS and HTM securities percentage	31.00%	41.00%
Corporate debt securities, information technology [Member]
Held to maturity securities:
AFS and HTM securities percentage	18.00%	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 of investments in debt securities classified as available-for-sale, expected to be converted to cash, sold or exchanged within one year or the normal operating cycle, if longer.

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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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