Fair Values of Assets and Liabilities - Balances of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Millions		Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities netting		$ (969)		$ (1,130)
Derivative assets netting		(899)		(652)
Available-for-sale securities	[1]	64,912		68,137
Mortgage loans held for sale		3,228		3,534
Mortgage servicing rights		2,917	$ 2,844	2,645	$ 2,598	$ 2,582	$ 2,591
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities total		1,069		1,237
Derivative assets total		975		1,830
Available-for-sale securities		64,912		68,137
Mortgage loans held for sale		3,228		3,534
Mortgage servicing rights		2,917		2,645
Other assets		1,710		1,317
Total		73,742		77,463
Short-term borrowings and other liabilities		1,256		995
Total		2,325		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		(969)		(1,130)
Derivative assets netting		(899)		(652)
Total		(899)		(652)
Total		(969)		(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		19,581		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		6,608		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		38,310		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		5		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		408		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		16		6
Available-for-sale securities		18,910		22,594
Other assets		216		154
Total		19,142		22,754
Short-term borrowings and other liabilities		205		101
Total		205		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		18,910		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		1,248		1,958
Derivative assets before netting		1,426		1,960
Available-for-sale securities		46,002		45,543
Mortgage loans held for sale		3,228		3,534
Other assets		1,494		1,163
Total		52,150		52,200
Short-term borrowings and other liabilities		1,051		894
Total		2,299		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		671		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		6,608		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		38,310		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		5		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		408		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		790		409
Derivative assets before netting		432		516
Mortgage servicing rights		2,917		2,645
Total		3,349		3,161
Total		$ 790		$ 409

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