Fair Value Measurements - Carrying Value and Estimated Fair Value of Financial Instruments (Detail) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Federal funds sold and securities purchased under agreements to resell	$ 3,760	$ 35,746
Loans, net of allowance for credit losses	60,673	61,073	$ 80,797
Loans held for sale	351	337
Deposits	148,582	145,150	$ 136,723
Long-term debt	18,179	19,979
Commercial Loans [Member]
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Loans held for sale	256	129
Consumer Loans [Member]
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Loans held for sale	95	208
Consumer Loans [Member] | Residential Mortgage [Member]
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Loans held for sale	95	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	1,447	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
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
Level 1 [Member] | Consumer Loans [Member] | Residential Mortgage [Member]
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
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	53,607	14,353
Federal funds sold and securities purchased under agreements to resell	3,760	35,746
Securities held-to-maturity, net of allowance for credit losses	8,166	9,369
Short-term financial liabilities	5,243	4,952
Deposits	144,821	141,001
Long-term debt	8,735	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	173	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
Level 2 [Member] | Consumer Loans [Member] | Residential Mortgage [Member]
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Loans held for sale	96	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	12	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	13	13
Deposits	0	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	41,303	41,417
Loans held for sale	0	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	19,508	19,865
Level 3 [Member] | Consumer Loans [Member] | Residential Mortgage [Member]
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Loans held for sale	0	0
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	55,066	15,667
Federal funds sold and securities purchased under agreements to resell	3,760	35,746
Securities held-to-maturity, net of allowance for credit losses	7,837	8,981
Short-term financial liabilities	5,256	4,965
Deposits	144,817	140,995
Long-term debt	8,309	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	40,482	40,785
Loans held for sale	173	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	20,161	20,256
Carrying Value [Member] | Consumer Loans [Member] | Residential Mortgage [Member]
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Loans held for sale	95	208
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	55,066	15,667
Federal funds sold and securities purchased under agreements to resell	3,760	35,746
Securities held-to-maturity, net of allowance for credit losses	8,166	9,369
Short-term financial liabilities	5,256	4,965
Deposits	144,821	141,001
Long-term debt	8,735	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	41,303	41,417
Loans held for sale	173	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	19,508	19,865
Estimate of fair value [Member] | Consumer Loans [Member] | Residential Mortgage [Member]
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Loans held for sale	$ 96	$ 217