Investment Securities - Amortized Cost, Gross Unrealized Gains and Losses, and Fair Value of Securities Available for Sale and Held to Maturity (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2024	Dec. 31, 2023
Available for sale securities
Amortized Cost	$ 109,627	$ 116,336
Gross Unrealized Gains	0	1
Gross Unrealized Losses	8,541	8,691
Fair Value	101,086	107,646
Unallocated portfolio layer method fair value basis adjustments, Amortized Cost	(47)	(19)
Unallocated portfolio layer method fair value basis adjustments, Gross Unrealized Gains	0	0
Unallocated portfolio layer method fair value basis adjustments, Gross Unrealized Losses	(47)	(19)
Held to maturity securities
Amortized Cost	156,371	159,452
Gross Unrealized Gains	402	1,435
Gross Unrealized Losses	14,433	13,796
Fair Value	142,340	147,091
U.S. agency mortgage-backed securities
Available for sale securities
Amortized Cost	66,573	69,173
Gross Unrealized Gains	0	0
Gross Unrealized Losses	6,402	6,378
Fair Value	60,171	62,795
Held to maturity securities
Amortized Cost	156,371	159,452
Gross Unrealized Gains	402	1,435
Gross Unrealized Losses	14,433	13,796
Fair Value	142,340	147,091
U.S. Treasury securities
Available for sale securities
Amortized Cost	21,095	22,459
Gross Unrealized Gains	0	1
Gross Unrealized Losses	902	989
Fair Value	20,193	21,471
Corporate debt securities
Available for sale securities
Amortized Cost	12,232	13,344
Gross Unrealized Gains	0	0
Gross Unrealized Losses	864	860
Fair Value	11,368	12,484
Asset-Backed Securities
Available for sale securities
Amortized Cost	8,166	9,465
Gross Unrealized Gains	0	0
Gross Unrealized Losses	319	378
Fair Value	7,847	9,087
Foreign government agency securities
Available for sale securities
Amortized Cost	831	1,035
Gross Unrealized Gains	0	0
Gross Unrealized Losses	28	33
Fair Value	803	1,002
U.S. state and municipal securities
Available for sale securities
Amortized Cost	632	634
Gross Unrealized Gains	0	0
Gross Unrealized Losses	57	55
Fair Value	575	579
Non-agency commercial mortgage-backed securities
Available for sale securities
Amortized Cost	123	123
Gross Unrealized Gains	0	0
Gross Unrealized Losses	13	14
Fair Value	110	109
Certificates of deposit
Available for sale securities
Amortized Cost		100
Gross Unrealized Gains		0
Gross Unrealized Losses		0
Fair Value		100
Other
Available for sale securities
Amortized Cost	22	22
Gross Unrealized Gains	0	0
Gross Unrealized Losses	3	3
Fair Value	$ 19	$ 19
Corporate Debt Securities Issued By Financial Services Industry
Held to maturity securities
AFS securities percentage	34.00%	36.00%
Federal Family Education Loan Program (FFELP) Guaranteed Loans \| Asset-Backed Securities \| Financial Instrument Concentration Risk
Held to maturity securities
Concentration risk percentage	57.00%	61.00%
Collateralized Credit Card Securities \| Asset-Backed Securities \| Financial Instrument Concentration Risk
Held to maturity securities
Concentration risk percentage	27.00%	24.00%

Concentration Risk, Available-For-Sale

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Hedged Asset, Fair Value Hedge, Portfolio Layer Method, Hedged Layer, Accumulated Gross Unrealized Gain, Before Tax, Cumulative Increase (Decrease)

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Hedged Asset, Fair Value Hedge, Portfolio Layer Method, Hedged Layer, Accumulated Gross Unrealized Loss, Before Tax, Cumulative Increase (Decrease)

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Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

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Amortized cost excluding accrued interest, after allowance for credit loss, of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount excluding accrued interest, of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount excluding accrued interest, after allowance for credit loss, of investment in debt security measured at amortized cost (held-to-maturity).

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Amount of cumulative increase (decrease) in fair value of hedged asset in fair value hedge, attributable to hedged risk, in portfolio layer hedging relationship.

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Amount of accumulated unrecognized gain on investment in debt security measured at amortized cost (held-to-maturity).

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Amount of accumulated unrealized loss on investment in debt security measured at amortized cost (held-to-maturity).

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Fair value of investment in debt security measured at amortized cost (held-to-maturity).

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