Fair Values of Assets and Liabilities - Balances of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Millions		Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities netting		$ (982)		$ (1,130)
Derivative liabilities total		889		499
Derivative assets netting		(881)		(652)
Derivative assets total		970		1,107
Available-for-sale securities	[1]	66,347		68,137
Mortgage loans held for sale		3,248		3,534
Mortgage servicing rights		2,844	$ 2,780	2,645	$ 2,582	$ 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		1,011		1,237
Derivative assets total		1,003		1,830
Available-for-sale securities		66,347		68,137
Mortgage loans held for sale		3,248		3,534
Mortgage servicing rights		2,844		2,645
Other assets		1,507		1,317
Total		74,949		77,463
Short-term borrowings and other liabilities		1,289		995
Total		2,300		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		(982)		(1,130)
Derivative assets netting		(881)		(652)
Total		(881)		(652)
Total		(982)		(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		20,731		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		6,779		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		38,420		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		6		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		411		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		20,039		22,594
Other assets		179		154
Total		20,235		22,754
Short-term borrowings and other liabilities		172		101
Total		172		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		20,039		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		1,264		1,958
Derivative assets before netting		1,394		1,960
Available-for-sale securities		46,308		45,543
Mortgage loans held for sale		3,248		3,534
Other assets		1,328		1,163
Total		52,278		52,200
Short-term borrowings and other liabilities		1,117		894
Total		2,381		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		692		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		6,779		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		38,420		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		6		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		411		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		729		409
Derivative assets before netting		473		516
Mortgage servicing rights		2,844		2,645
Total		3,317		3,161
Total		$ 729		$ 409

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