Fair Values of Assets and Liabilities - Balances of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Millions		Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities netting		$ (1,189)		$ (1,185)
Derivative liabilities total		359		766
Derivative assets netting		(810)		(984)
Derivative assets total		942		1,138
Available-for-sale securities	[1]	67,455		66,284
Mortgage loans held for sale		3,656		4,822
Mortgage servicing rights		2,582	$ 2,642	2,591	$ 2,056	$ 2,222	$ 2,512
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities total		1,062		1,674
Derivative assets total		1,509		1,986
Available-for-sale securities		67,455		66,284
Mortgage loans held for sale		3,656		4,822
Mortgage servicing rights		2,582		2,591
Other assets		1,565		1,320
Total		76,767		77,003
Short-term borrowings and other liabilities		1,186		1,080
Total		2,248		2,754
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		(1,189)		(1,185)
Derivative assets netting		(810)		(984)
Total		(810)		(984)
Total		(1,189)		(1,185)
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		20,539		17,127
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		5,469		5,039
Fair Value, Measurements, Recurring [Member] | Corporate Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities				9
Fair Value, Measurements, Recurring [Member] | Other Investment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		34		36
Fair Value, Measurements, Recurring [Member] | Mortgage-Backed Securities Residential [Member] | Non-Agency Prime [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities				242
Fair Value, Measurements, Recurring [Member] | Mortgage-Backed Securities Residential [Member] | Non-Agency Non-Prime [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities				195
Fair Value, Measurements, Recurring [Member] | Mortgage-Backed Securities Residential [Member] | Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		40,966		43,138
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		10		15
Fair Value, Measurements, Recurring [Member] | Asset-Backed Securities [Member] | Asset-Backed Securities Other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		437		483
Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities before netting		1		7
Available-for-sale securities		19,812		16,391
Other assets		288		183
Total		20,100		16,574
Short-term borrowings and other liabilities		125		142
Total		126		149
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		19,778		16,355
Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Other Investment [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		34		36
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		1,925		2,469
Derivative assets before netting		1,754		2,416
Available-for-sale securities		47,643		49,445
Mortgage loans held for sale		3,656		4,822
Other assets		1,277		1,137
Total		54,330		57,820
Short-term borrowings and other liabilities		1,061		938
Total		2,986		3,407
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		761		772
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		5,469		5,039
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Mortgage-Backed Securities Residential [Member] | Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		40,966		43,138
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		10		15
Level 2 [Member] | Fair Value, Measurements, Recurring [Member] | Asset-Backed Securities [Member] | Asset-Backed Securities Other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		437		481
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		325		383
Derivative assets before netting		565		554
Available-for-sale securities				448
Mortgage servicing rights		2,582		2,591
Total		3,147		3,593
Total		$ 325		383
Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Corporate Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities				9
Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Mortgage-Backed Securities Residential [Member] | Non-Agency Prime [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities				242
Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Mortgage-Backed Securities Residential [Member] | Non-Agency Non-Prime [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities				195
Level 3 [Member] | Fair Value, Measurements, Recurring [Member] | Asset-Backed Securities [Member] | Asset-Backed Securities Other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities				$ 2

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