Asset Quality - Breakdown of Nonperforming TDRs by Loans Category (Detail) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2017 USD ($) SecurityLoan	Dec. 31, 2016 USD ($) SecurityLoan
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	5,057
Pre-modification Outstanding Recorded Investment	$ 396
Post-modification Outstanding Recorded Investment	$ 315
Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan		1,980
Pre-modification Outstanding Recorded Investment		$ 193
Post-modification Outstanding Recorded Investment		$ 141
Commercial Loans [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan		25
Pre-modification Outstanding Recorded Investment		$ 93
Post-modification Outstanding Recorded Investment		$ 51
Consumer Portfolio Segment [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan		1,955
Pre-modification Outstanding Recorded Investment		$ 100
Post-modification Outstanding Recorded Investment		$ 90
Nonperforming [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	2,505
Pre-modification Outstanding Recorded Investment	$ 225
Post-modification Outstanding Recorded Investment	$ 187
Prior-Year Accruing [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	2,552
Pre-modification Outstanding Recorded Investment	$ 171
Post-modification Outstanding Recorded Investment	$ 128
Prior-Year Accruing [Member] | Commercial Loans [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan		35
Pre-modification Outstanding Recorded Investment		$ 2
Post-modification Outstanding Recorded Investment		$ 2
Commercial and Industrial [Member] | Commercial Loans [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan		18
Pre-modification Outstanding Recorded Investment		$ 91
Post-modification Outstanding Recorded Investment		$ 50
Commercial and Industrial [Member] | Nonperforming [Member] | Commercial Loans [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	20
Pre-modification Outstanding Recorded Investment	$ 105
Post-modification Outstanding Recorded Investment	$ 83
Commercial and Industrial [Member] | Prior-Year Accruing [Member] | Commercial Loans [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	5	5
Pre-modification Outstanding Recorded Investment	$ 30	$ 30
Post-modification Outstanding Recorded Investment	$ 13	$ 16
Commercial Loans [Member] | Nonperforming [Member] | Commercial Loans [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	28
Pre-modification Outstanding Recorded Investment	$ 122
Post-modification Outstanding Recorded Investment	$ 95
Commercial Loans [Member] | Prior-Year Accruing [Member] | Commercial Loans [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	5	5
Pre-modification Outstanding Recorded Investment	$ 30	$ 30
Post-modification Outstanding Recorded Investment	$ 13	$ 16
Commercial Real Estate: Commercial Mortgage [Member] | Commercial Loans [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan		7
Pre-modification Outstanding Recorded Investment		$ 2
Post-modification Outstanding Recorded Investment		$ 1
Commercial Real Estate: Commercial Mortgage [Member] | Nonperforming [Member] | Commercial Loans [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	8
Pre-modification Outstanding Recorded Investment	$ 17
Post-modification Outstanding Recorded Investment	$ 12
Commercial Real Estate: Commercial Mortgage [Member] | Prior-Year Accruing [Member] | Commercial Loans [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan		477
Pre-modification Outstanding Recorded Investment		$ 35
Post-modification Outstanding Recorded Investment		$ 35
Commercial Real Estate Loans [Member] | Commercial Loans [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan		7
Pre-modification Outstanding Recorded Investment		$ 2
Post-modification Outstanding Recorded Investment		$ 1
Commercial Real Estate Loans [Member] | Nonperforming [Member] | Commercial Loans [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	8
Pre-modification Outstanding Recorded Investment	$ 17
Post-modification Outstanding Recorded Investment	$ 12
Commercial Real Estate Loans [Member] | Prior-Year Accruing [Member] | Commercial Loans [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan		1,231
Pre-modification Outstanding Recorded Investment		$ 70
Post-modification Outstanding Recorded Investment		$ 57
Real Estate - Residential Mortgage [Member] | Consumer Portfolio Segment [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan		264
Pre-modification Outstanding Recorded Investment		$ 16
Post-modification Outstanding Recorded Investment		$ 16
Real Estate - Residential Mortgage [Member] | Nonperforming [Member] | Consumer Portfolio Segment [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	303
Pre-modification Outstanding Recorded Investment	$ 18
Post-modification Outstanding Recorded Investment	$ 18
Real Estate - Residential Mortgage [Member] | Prior-Year Accruing [Member] | Consumer Portfolio Segment [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	499	410
Pre-modification Outstanding Recorded Investment	$ 32	$ 3
Post-modification Outstanding Recorded Investment	$ 32	$ 1
Home Equity Loans [Member] | Consumer Portfolio Segment [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	1,145	1,199
Pre-modification Outstanding Recorded Investment	$ 69	$ 77
Post-modification Outstanding Recorded Investment	$ 61	$ 69
Home Equity Loans [Member] | Prior-Year Accruing [Member] | Consumer Portfolio Segment [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	1,223	377
Pre-modification Outstanding Recorded Investment	$ 72	$ 56
Post-modification Outstanding Recorded Investment	$ 57	$ 28
Consumer Direct Loans [Member] | Consumer Portfolio Segment [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan		32
Pre-modification Outstanding Recorded Investment		$ 1
Post-modification Outstanding Recorded Investment		$ 0
Consumer Direct Loans [Member] | Nonperforming [Member] | Consumer Portfolio Segment [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	55
Pre-modification Outstanding Recorded Investment	$ 1
Post-modification Outstanding Recorded Investment	$ 1
Consumer Direct Loans [Member] | Prior-Year Accruing [Member] | Consumer Portfolio Segment [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	24	2,530
Pre-modification Outstanding Recorded Investment	$ 1	$ 166
Post-modification Outstanding Recorded Investment	$ 1	$ 123
Consumer Credit Card [Member] | Consumer Portfolio Segment [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan		336
Pre-modification Outstanding Recorded Investment		$ 2
Post-modification Outstanding Recorded Investment		$ 2
Consumer Credit Card [Member] | Nonperforming [Member] | Consumer Portfolio Segment [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	250
Pre-modification Outstanding Recorded Investment	$ 1
Post-modification Outstanding Recorded Investment	$ 1
Consumer Credit Card [Member] | Prior-Year Accruing [Member] | Consumer Portfolio Segment [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	480	2,535
Pre-modification Outstanding Recorded Investment	$ 3	$ 196
Post-modification Outstanding Recorded Investment	$ 2	$ 139
Consumer Indirect Loans [Member] | Consumer Portfolio Segment [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	724	124
Pre-modification Outstanding Recorded Investment	$ 14	$ 4
Post-modification Outstanding Recorded Investment	$ 11	$ 3
Consumer Indirect Loans [Member] | Prior-Year Accruing [Member] | Consumer Portfolio Segment [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	321	4,515
Pre-modification Outstanding Recorded Investment	$ 33	$ 389
Post-modification Outstanding Recorded Investment	$ 23	$ 280
Consumer Portfolio Segment [Member] | Nonperforming [Member] | Consumer Portfolio Segment [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	2,477
Pre-modification Outstanding Recorded Investment	$ 103
Post-modification Outstanding Recorded Investment	$ 92
Consumer Portfolio Segment [Member] | Prior-Year Accruing [Member] | Consumer Portfolio Segment [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	2,547
Pre-modification Outstanding Recorded Investment	$ 141
Post-modification Outstanding Recorded Investment	$ 115