Fair Values of Assets and Liabilities - Balances of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Millions		Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities netting		$ (1,182)		$ (1,185)
Derivative liabilities total		399		766
Derivative assets netting		(727)		(984)
Derivative assets total		1,117		1,138
Available-for-sale securities	[1]	67,772		66,284
Mortgage loans held for sale		3,754		4,822
Mortgage servicing rights		2,598	$ 2,582	2,591	$ 2,131	$ 2,056	$ 2,512
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities total		1,167		1,674
Derivative assets total		1,680		1,986
Available-for-sale securities		67,772		66,284
Mortgage loans held for sale		3,754		4,822
Mortgage servicing rights		2,598		2,591
Other assets		1,560		1,320
Total		77,364		77,003
Short-term borrowings and other liabilities		1,215		1,080
Total		2,382		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		(1,182)		(1,185)
Derivative assets netting		(727)		(984)
Other assets		0
Total		(727)		(984)
Total		(1,182)		(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		22,068		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		5,681		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		37		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		39,553		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		8		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		425		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				7
Derivative assets before netting		1
Available-for-sale securities		21,358		16,391
Other assets		258		183
Total		21,617		16,574
Short-term borrowings and other liabilities		138		142
Total		138		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		21,321		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		37		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		2,032		2,469
Derivative assets before netting		1,836		2,416
Available-for-sale securities		46,414		49,445
Mortgage loans held for sale		3,754		4,822
Other assets		1,302		1,137
Total		53,306		57,820
Short-term borrowings and other liabilities		1,077		938
Total		3,109		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		747		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		5,681		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		39,553		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		8		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		425		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		317		383
Derivative assets before netting		570		554
Available-for-sale securities				448
Mortgage servicing rights		2,598		2,591
Total		3,168		3,593
Total		$ 317		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.