Fair Values of Assets and Liabilities - Balances of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Millions		Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities netting		$ (796)	$ (946)
Derivative assets netting		(872)	(942)
Available-for-sale securities	[1]	68,113	66,115
Mortgage loans held for sale		2,690	2,035
Mortgage servicing rights		2,656	2,791	$ 2,780	$ 2,645
Residential Mortgage-Backed Securities [Member] | Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		41,879	39,752
Commercial [Member] | Agency [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		2	2
Asset-Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities		395
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		395	403
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative liabilities total		811	849
Derivative assets total		1,345	1,068
Available-for-sale securities		68,113	66,115
Mortgage loans held for sale		2,690	2,035
Mortgage servicing rights		2,656	2,791
Other assets		1,973	1,665
Total		76,777	73,674
Short-term borrowings and other liabilities		1,472	1,218
Total		2,283	2,067
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		(796)	(946)
Derivative assets netting		(872)	(942)
Total		(872)	(942)
Total		(796)	(946)
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		18,919	19,257
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		6,918	6,701
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		41,879	39,752
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		2	2
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		395	403
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		8	1
Available-for-sale securities		18,266	18,585
Other assets		249	392
Total		18,515	18,977
Short-term borrowings and other liabilities		189	199
Total		197	200
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		18,266	18,585
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		1,242	1,291
Derivative assets before netting		1,405	1,427
Available-for-sale securities		49,847	47,530
Mortgage loans held for sale		2,690	2,035
Other assets		1,724	1,273
Total		55,666	52,265
Short-term borrowings and other liabilities		1,283	1,019
Total		2,525	2,310
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		653	672
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		6,918	6,701
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		41,879	39,752
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		2	2
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		395	403
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		357	503
Derivative assets before netting		812	583
Mortgage servicing rights		2,656	2,791
Total		3,468	3,374
Total		$ 357	$ 503

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