Fair Value Measurements - Carrying Value and Estimated Fair Value of Financial Instruments (Detail) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Federal funds sold and securities purchased under agreements to resell	$ 7,480	$ 35,746
Loans, net of allowance for credit losses	55,051		$ 65,325
Loans held for sale	4,927	337
Deposits	152,368	145,150	$ 150,328
Long-term debt	18,105	19,979
Commercial Loans [Member]
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Loans held for sale	1,291	129
Consumer Loans [Member]
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Loans held for sale	3,636	208
Level 1 [Member]
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Short-term financial assets, net of allowance for credit losses	1,018	1,302
Federal funds sold and securities purchased under agreements to resell	0	0
Securities held-to-maturity, net of allowance for credit losses	0	0
Short-term financial liabilities	0	0
Deposits	0	0
Deposits held for sale	0
Long-term debt	0	0
Level 1 [Member] \| Commercial Loans [Member]
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Loans, net of allowance for credit losses	0	0
Loans held for sale	0	0
Level 1 [Member] \| Consumer Loans [Member]
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Loans, net of allowance for credit losses	0	0
Loans held for sale	0	0
Level 2 [Member]
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Short-term financial assets, net of allowance for credit losses	62,140	14,353
Federal funds sold and securities purchased under agreements to resell	7,480	35,746
Securities held-to-maturity, net of allowance for credit losses	6,097	9,369
Short-term financial liabilities	6,377	4,952
Deposits	140,072	141,001
Deposits held for sale	9,321
Long-term debt	8,878	9,720
Level 2 [Member] \| Commercial Loans [Member]
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Loans, net of allowance for credit losses	0	0
Loans held for sale	0	93
Level 2 [Member] \| Consumer Loans [Member]
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Loans, net of allowance for credit losses	0	0
Loans held for sale	53	217
Level 3 [Member]
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Short-term financial assets, net of allowance for credit losses	46	12
Federal funds sold and securities purchased under agreements to resell	0	0
Securities held-to-maturity, net of allowance for credit losses	0	0
Short-term financial liabilities	48	13
Deposits	0	0
Deposits held for sale	0
Long-term debt	0	0
Level 3 [Member] \| Commercial Loans [Member]
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Loans, net of allowance for credit losses	39,026	41,417
Loans held for sale	1,231	0
Level 3 [Member] \| Consumer Loans [Member]
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Loans, net of allowance for credit losses	16,140	19,865
Loans held for sale	3,685	0
Estimate of fair value [Member]
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Short-term financial assets, net of allowance for credit losses	63,204	15,667
Federal funds sold and securities purchased under agreements to resell	7,480	35,746
Securities held-to-maturity, net of allowance for credit losses	6,097	9,369
Short-term financial liabilities	6,425	4,965
Deposits	140,072	141,001
Deposits held for sale	9,321
Long-term debt	8,878	9,720
Estimate of fair value [Member] \| Commercial Loans [Member]
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Loans, net of allowance for credit losses	39,026	41,417
Loans held for sale	1,231	93
Estimate of fair value [Member] \| Consumer Loans [Member]
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Loans, net of allowance for credit losses	16,140	19,865
Loans held for sale	3,738	217
Carrying Value [Member]
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Short-term financial assets, net of allowance for credit losses	63,204	15,667
Federal funds sold and securities purchased under agreements to resell	7,480	35,746
Securities held-to-maturity, net of allowance for credit losses	5,866	8,981
Short-term financial liabilities	6,425	4,965
Deposits	140,077	140,995
Deposits held for sale	9,321
Long-term debt	8,370	9,254
Carrying Value [Member] \| Commercial Loans [Member]
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Loans, net of allowance for credit losses	38,439	40,785
Loans held for sale	1,228	93
Carrying Value [Member] \| Consumer Loans [Member]
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Loans, net of allowance for credit losses	16,585	20,256
Loans held for sale	$ 3,636	$ 208

Debt Securities, Held-to-maturity, Net of Allowance for Credit Losses

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Deposits Held for Sale

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Short Term Financial Assets, Net of Allowance for Credit Losses

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Short-term Financial Liabilities

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The aggregate of all deposit liabilities held by the entity, including foreign and domestic, interest and noninterest bearing; may include demand deposits, saving deposits, Negotiable Order of Withdrawal (NOW) and time deposits among others.

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Includes: (1) the amount outstanding of funds lent to other depository institutions, securities brokers, or securities dealers in the form of Federal Funds sold; for example, immediately available funds lent under agreements or contracts that mature in one business day or roll over under a continuing contract, regardless of the nature of the transaction or the collateral involved, excluding overnight lending for commercial and industrial purposes. Also include Federal Funds sold under agreements to resell on a gross basis, excluding (1) sales of term Federal Funds, (2) due bills representing purchases of securities or other assets by the reporting bank that have not yet been delivered and similar instruments, (3) resale agreements that mature in more than one business day involving assets other than securities, and (4) yield maintenance dollar repurchase agreements (Federal Funds Sold) and (2) the dollar amount outstanding of funds lent in the form of security resale agreements regardless of maturity, if the agreement requires the bank to resell the identical security purchased or a security that meets the definition of substantially the same in the case of a dollar roll. Also include purchases of participations in pools of securities, regardless of maturity (Securities Purchased Under Agreements to Re-sell).

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Amount, before valuation allowance, of financing receivable classified as held-for-sale.

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Amount, after allowance for credit loss, of financing receivable. Excludes financing receivable covered under loss sharing agreement.

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