Fair Values of Assets and Liabilities - Balances of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Millions		Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities netting		$ (1,328)	$ (1,130)
Derivative liabilities total		795	499
Derivative assets netting		(821)	(652)
Derivative assets total		1,273	1,107
Available-for-sale securities	[1]	67,125	68,137
Mortgage loans held for sale		3,271	3,534
Mortgage servicing rights		2,780	2,645	$ 2,642	$ 2,591
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities total		924	1,237
Derivative assets total		1,309	1,830
Available-for-sale securities		67,125	68,137
Mortgage loans held for sale		3,271	3,534
Mortgage servicing rights		2,780	2,645
Other assets		1,814	1,317
Total		76,299	77,463
Short-term borrowings and other liabilities		1,350	995
Total		2,274	2,232
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		(1,328)	(1,130)
Derivative assets netting		(821)	(652)
Total		(821)	(652)
Total		(1,328)	(1,130)
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		21,966	23,301
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		6,294	6,358
Fair Value, Measurements, Recurring [Member] | Other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities			22
Fair Value, Measurements, Recurring [Member] | Residential Mortgage-Backed Securities [Member] | Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		38,443	38,031
Fair Value, Measurements, Recurring [Member] | Commercial [Member] | Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		7	6
Fair Value, Measurements, Recurring [Member] | Asset-Backed Securities [Member] | Other Asset-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		415	419
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		17	6
Available-for-sale securities		21,258	22,594
Other assets		199	154
Total		21,474	22,754
Short-term borrowings and other liabilities		183	101
Total		183	101
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		21,258	22,572
Level 1 [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			22
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		1,618	1,958
Derivative assets before netting		1,611	1,960
Available-for-sale securities		45,867	45,543
Mortgage loans held for sale		3,271	3,534
Other assets		1,615	1,163
Total		52,364	52,200
Short-term borrowings and other liabilities		1,167	894
Total		2,785	2,852
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		708	729
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		6,294	6,358
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Residential Mortgage-Backed Securities [Member] | Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		38,443	38,031
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Commercial [Member] | Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		7	6
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Asset-Backed Securities [Member] | Other Asset-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		415	419
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		634	409
Derivative assets before netting		502	516
Mortgage servicing rights		2,780	2,645
Total		3,282	3,161
Total		$ 634	$ 409

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