Fair Values of Assets and Liabilities - Balances of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Millions		Sep. 30, 2021		Jun. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities netting		$ (1,697)			$ (2,312)
Derivative assets netting		(1,858)			(1,874)
Available-for-sale securities	[1]	149,376	[2]		136,840
Mortgage loans held for sale		6,176			8,524
Mortgage servicing rights		2,910		$ 2,713	2,210	$ 1,978	$ 1,840	$ 2,546
Asset-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		69	[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		1,285			1,290
Derivative assets total		2,510			4,127
Available-for-sale securities		149,376			136,840
Mortgage loans held for sale		6,176			8,524
Mortgage servicing rights		2,910			2,210
Other assets		2,440			1,903
Total		163,412			153,604
Short-term borrowings and other liabilities		2,098			1,757
Total		3,383			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		(1,697)			(2,312)
Derivative assets netting		(1,858)			(1,874)
Total		(1,858)			(1,874)
Total		(1,697)			(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		18,491			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		10,327			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		112,796			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		7,686			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		69			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		6			4
Available-for-sale securities		14,344			19,251
Other assets		322			302
Total		14,672			19,557
Short-term borrowings and other liabilities		205			85
Total		205			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		14,344			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		2,429			3,166
Derivative assets before netting		2,691			3,235
Available-for-sale securities		135,023			117,581
Mortgage loans held for sale		6,176			8,524
Other assets		2,118			1,601
Total		146,008			130,941
Short-term borrowings and other liabilities		1,893			1,672
Total		4,322			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		4,147			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		10,326			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		112,796			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		7,686			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		61			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		553			436
Derivative assets before netting		1,671			2,762
Available-for-sale securities		9			8
Mortgage servicing rights		2,910			2,210
Total		4,590			4,980
Total		553			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		$ 8			$ 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.