Fair Values of Assets and Liabilities - Balances of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Millions		Mar. 31, 2021		Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities netting		$ (2,084)		$ (2,312)
Derivative assets netting		(1,938)		(1,874)
Available-for-sale securities	[1]	156,003	[2]	136,840
Mortgage loans held for sale		8,869		8,524
Mortgage servicing rights		2,787		2,210	$ 1,887	$ 2,546
Asset-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		202	[3]	205
Other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		7		9
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities total		1,402		1,290
Derivative assets total		2,956		4,127
Available-for-sale securities		156,003		136,840
Mortgage loans held for sale		8,869		8,524
Mortgage servicing rights		2,787		2,210
Other assets		2,149		1,903
Total		172,764		153,604
Short-term borrowings and other liabilities		2,089		1,757
Total		3,491		3,047
Fair Value, Measurements, Recurring [Member] | Netting and Collateral One [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities netting		(2,084)		(2,312)
Derivative assets netting		(1,938)		(1,874)
Total		(1,938)		(1,874)
Total		(2,084)		(2,312)
Fair Value, Measurements, Recurring [Member] | U.S. Treasury and Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		24,317		22,391
Fair Value, Measurements, Recurring [Member] | Obligations of State and Political Subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		9,215		8,861
Fair Value, Measurements, Recurring [Member] | Residential Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		116,176		99,968
Fair Value, Measurements, Recurring [Member] | Commercial Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		6,086		5,406
Fair Value, Measurements, Recurring [Member] | Asset-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		202		205
Fair Value, Measurements, Recurring [Member] | Other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		7		9
Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative assets before netting		7		4
Available-for-sale securities		21,406		19,251
Other assets		120		302
Total		21,533		19,557
Short-term borrowings and other liabilities		199		85
Total		199		85
Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | U.S. Treasury and Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		21,406		19,251
Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities before netting		2,821		3,166
Derivative assets before netting		3,066		3,235
Available-for-sale securities		134,589		117,581
Mortgage loans held for sale		8,869		8,524
Other assets		2,029		1,601
Total		148,553		130,941
Short-term borrowings and other liabilities		1,890		1,672
Total		4,711		4,838
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | U.S. Treasury and Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		2,911		3,140
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Obligations of State and Political Subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		9,214		8,860
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Residential Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		116,176		99,968
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Commercial Mortgage-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		6,086		5,406
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Asset-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		195		198
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		7		9
Level 3 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities before netting		665		436
Derivative assets before netting		1,821		2,762
Available-for-sale securities		8		8
Mortgage servicing rights		2,787		2,210
Total		4,616		4,980
Total		665		436
Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Obligations of State and Political Subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		1		1
Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Asset-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		$ 7		$ 7

[1] Includes only collateral pledged by the Company where counterparties have the right to sell or pledge the collateral.

[2] The weighted-average maturity of total available-for-sale investment securities was 3.4 years at December 31, 2020, with a corresponding weighted-average yield of 1.61 percent.

[3] Information related to asset and mortgage-backed securities included above is presented based upon weighted-average maturities that take into account anticipated future prepayments.