Derivatives (Notional Amounts and Fair Values for All Derivative Instruments Included in the Consolidated Balance Sheets) (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Derivatives, Fair Value
Fair value - Derivative Assets	$ 823	$ 1,057
Fair value - Derivative Liabilities	602	666
Accumulated Net Gain (Loss) from Designated or Qualifying Hedges
Derivatives, Fair Value
Fair value - Derivative Assets	297	345
Fair value - Derivative Liabilities	17	12
Accumulated Net Gain (Loss) from Designated or Qualifying Hedges | Fair Value Hedging
Derivatives, Fair Value
Fair value - Derivative Assets	297	323
Fair value - Derivative Liabilities	5	12
Accumulated Net Gain (Loss) from Designated or Qualifying Hedges | Fair Value Hedging | Interest Rate Swap | Long-Term Debt
Derivatives, Fair Value
Notional amount	3,705	3,455
Fair value - Derivative Assets	297	323
Fair value - Derivative Liabilities	5	12
Accumulated Net Gain (Loss) from Designated or Qualifying Hedges | Cash Flow Hedging
Derivatives, Fair Value
Fair value - Derivative Assets	0	22
Fair value - Derivative Liabilities	12	0
Accumulated Net Gain (Loss) from Designated or Qualifying Hedges | Cash Flow Hedging | Interest Rate Swap | Commercial and Industrial Loans
Derivatives, Fair Value
Notional amount	4,475	4,475
Fair value - Derivative Assets	0	22
Fair value - Derivative Liabilities	12	0
Nondesignated
Derivatives, Fair Value
Fair value - Derivative Assets	526	712
Fair value - Derivative Liabilities	585	654
Nondesignated | Risk Management and Other Business Purposes
Derivatives, Fair Value
Fair value - Derivative Assets	75	185
Fair value - Derivative Liabilities	154	133
Nondesignated | Risk Management and Other Business Purposes | Interest Rate Contract | Mortgage Servicing Rights
Derivatives, Fair Value
Notional amount	11,035	10,522
Fair value - Derivative Assets	54	165
Fair value - Derivative Liabilities	15	39
Nondesignated | Risk Management and Other Business Purposes | Forward Contracts | Assets Held For Sale | Residential Mortgage
Derivatives, Fair Value
Notional amount	1,284	1,823
Fair value - Derivative Assets	1	20
Fair value - Derivative Liabilities	1	3
Nondesignated | Risk Management and Other Business Purposes | Warrant
Derivatives, Fair Value
Notional amount	20
Fair value - Derivative Assets	20
Fair value - Derivative Liabilities	0
Nondesignated | Risk Management and Other Business Purposes | Swap
Derivatives, Fair Value
Notional amount	1,900	1,300
Fair value - Derivative Assets	0	0
Fair value - Derivative Liabilities	137	91
Nondesignated | Risk Management and Other Business Purposes | TBA Securities
Derivatives, Fair Value
Notional amount	26
Fair value - Derivative Assets	0
Fair value - Derivative Liabilities	0
Nondesignated | Risk Management and Other Business Purposes | Foreign Exchange Contract
Derivatives, Fair Value
Notional amount	112	111
Fair value - Derivative Assets	0	0
Fair value - Derivative Liabilities	1	0
Nondesignated | Customer Accommodation
Derivatives, Fair Value
Fair value - Derivative Assets	451	527
Fair value - Derivative Liabilities	431	521
Nondesignated | Customer Accommodation | Interest Rate Contract
Derivatives, Fair Value
Notional amount	42,216	33,431
Fair value - Derivative Assets	154	205
Fair value - Derivative Liabilities	145	210
Nondesignated | Customer Accommodation | Interest Rate Lock Commitments
Derivatives, Fair Value
Notional amount	446	701
Fair value - Derivative Assets	8	13
Fair value - Derivative Liabilities	0	1
Nondesignated | Customer Accommodation | Commodity Contract
Derivatives, Fair Value
Notional amount	4,125	2,095
Fair value - Derivative Assets	165	107
Fair value - Derivative Liabilities	167	106
Nondesignated | Customer Accommodation | Foreign Exchange Contract
Derivatives, Fair Value
Notional amount	12,654	11,013
Fair value - Derivative Assets	124	202
Fair value - Derivative Liabilities	$ 119	$ 204