Asset Quality - Breakdown of Nonperforming TDRs by Loans Category (Details) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2018 USD ($) SecurityLoan	Dec. 31, 2017 USD ($) SecurityLoan
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	5,831	5,184
Pre-modification Outstanding Recorded Investment	$ 452	$ 398
Post-modification Outstanding Recorded Investment	$ 366	$ 317
Nonperforming [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	2,885	2,622
Pre-modification Outstanding Recorded Investment	$ 244	$ 227
Post-modification Outstanding Recorded Investment	$ 211	$ 189
Nonperforming [Member] | Commercial Loans [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	36	28
Pre-modification Outstanding Recorded Investment	$ 134	$ 125
Post-modification Outstanding Recorded Investment	$ 111	$ 98
Nonperforming [Member] | Consumer Portfolio Segment [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	2,849	2,594
Pre-modification Outstanding Recorded Investment	$ 110	$ 102
Post-modification Outstanding Recorded Investment	$ 100	$ 91
Prior-Year Accruing [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	2,946	2,562
Pre-modification Outstanding Recorded Investment	$ 208	$ 171
Post-modification Outstanding Recorded Investment	$ 155	$ 128
Prior-Year Accruing [Member] | Commercial Loans [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	18	4
Pre-modification Outstanding Recorded Investment	$ 53	$ 30
Post-modification Outstanding Recorded Investment	$ 37	$ 13
Prior-Year Accruing [Member] | Consumer Portfolio Segment [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	2,928	2,558
Pre-modification Outstanding Recorded Investment	$ 155	$ 141
Post-modification Outstanding Recorded Investment	$ 118	$ 115
Commercial and Industrial [Member] | Nonperforming [Member] | Commercial Loans [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	30	20
Pre-modification Outstanding Recorded Investment	$ 120	$ 109
Post-modification Outstanding Recorded Investment	$ 101	$ 86
Commercial and Industrial [Member] | Prior-Year Accruing [Member] | Commercial Loans [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	18	4
Pre-modification Outstanding Recorded Investment	$ 53	$ 30
Post-modification Outstanding Recorded Investment	$ 37	$ 13
Commercial Real Estate: Commercial Mortgage [Member] | Nonperforming [Member] | Commercial Real Estate Portfolio Segment [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	6	8
Pre-modification Outstanding Recorded Investment	$ 14	$ 16
Post-modification Outstanding Recorded Investment	$ 10	$ 12
Commercial Real Estate Loans [Member] | Nonperforming [Member] | Commercial Real Estate Portfolio Segment [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	6	8
Pre-modification Outstanding Recorded Investment	$ 14	$ 16
Post-modification Outstanding Recorded Investment	$ 10	$ 12
Real Estate - Residential Mortgage [Member] | Nonperforming [Member] | Consumer Portfolio Segment [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	257	308
Pre-modification Outstanding Recorded Investment	$ 18	$ 18
Post-modification Outstanding Recorded Investment	$ 18	$ 18
Real Estate - Residential Mortgage [Member] | Prior-Year Accruing [Member] | Consumer Portfolio Segment [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	508	484
Pre-modification Outstanding Recorded Investment	$ 37	$ 31
Post-modification Outstanding Recorded Investment	$ 31	$ 31
Home Equity Loans [Member] | Consumer Portfolio Segment [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	1,233	1,025
Pre-modification Outstanding Recorded Investment	$ 72	$ 64
Post-modification Outstanding Recorded Investment	$ 66	$ 57
Home Equity Loans [Member] | Prior-Year Accruing [Member] | Consumer Portfolio Segment [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	1,305	1,276
Pre-modification Outstanding Recorded Investment	$ 77	$ 75
Post-modification Outstanding Recorded Investment	$ 61	$ 59
Consumer Direct Loans [Member] | Nonperforming [Member] | Consumer Portfolio Segment [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	137	114
Pre-modification Outstanding Recorded Investment	$ 2	$ 2
Post-modification Outstanding Recorded Investment	$ 2	$ 2
Consumer Direct Loans [Member] | Prior-Year Accruing [Member] | Consumer Portfolio Segment [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	81	48
Pre-modification Outstanding Recorded Investment	$ 4	$ 3
Post-modification Outstanding Recorded Investment	$ 2	$ 2
Consumer Credit Card [Member] | Nonperforming [Member] | Consumer Portfolio Segment [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	249	322
Pre-modification Outstanding Recorded Investment	$ 1	$ 2
Post-modification Outstanding Recorded Investment	$ 1	$ 1
Consumer Credit Card [Member] | Prior-Year Accruing [Member] | Consumer Portfolio Segment [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	522	430
Pre-modification Outstanding Recorded Investment	$ 3	$ 1
Post-modification Outstanding Recorded Investment	$ 2	$ 1
Consumer Indirect Loans [Member] | Consumer Portfolio Segment [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	973	825
Pre-modification Outstanding Recorded Investment	$ 17	$ 16
Post-modification Outstanding Recorded Investment	$ 13	$ 13
Consumer Indirect Loans [Member] | Prior-Year Accruing [Member] | Consumer Portfolio Segment [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	512	320
Pre-modification Outstanding Recorded Investment	$ 34	$ 31
Post-modification Outstanding Recorded Investment	$ 22	$ 22