Asset Quality - Breakdown of Nonperforming TDRs by Loans Category (Details) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2018 USD ($) SecurityLoan	Dec. 31, 2017 USD ($) SecurityLoan
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	5,706	5,184
Pre-modification Outstanding Recorded Investment	$ 434	$ 398
Post-modification Outstanding Recorded Investment	$ 347	$ 317
Nonperforming [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	2,635	2,622
Pre-modification Outstanding Recorded Investment	$ 216	$ 227
Post-modification Outstanding Recorded Investment	$ 184	$ 189
Nonperforming [Member] | Commercial Loans [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	50	28
Pre-modification Outstanding Recorded Investment	$ 119	$ 125
Post-modification Outstanding Recorded Investment	$ 95	$ 98
Nonperforming [Member] | Consumer Portfolio Segment [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	2,585	2,594
Pre-modification Outstanding Recorded Investment	$ 97	$ 102
Post-modification Outstanding Recorded Investment	$ 89	$ 91
Prior-Year Accruing [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	3,071	2,562
Pre-modification Outstanding Recorded Investment	$ 218	$ 171
Post-modification Outstanding Recorded Investment	$ 163	$ 128
Prior-Year Accruing [Member] | Commercial Loans [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	18	4
Pre-modification Outstanding Recorded Investment	$ 54	$ 30
Post-modification Outstanding Recorded Investment	$ 37	$ 13
Prior-Year Accruing [Member] | Commercial Real Estate Portfolio Segment [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	0	0
Pre-modification Outstanding Recorded Investment	$ 0	$ 0
Post-modification Outstanding Recorded Investment	$ 0	$ 0
Prior-Year Accruing [Member] | Consumer Portfolio Segment [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	3,053	2,558
Pre-modification Outstanding Recorded Investment	$ 164	$ 141
Post-modification Outstanding Recorded Investment	$ 126	$ 115
Commercial and Industrial [Member] | Nonperforming [Member] | Commercial Loans [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	44	20
Pre-modification Outstanding Recorded Investment	$ 106	$ 109
Post-modification Outstanding Recorded Investment	$ 86	$ 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	$ 54	$ 30
Post-modification Outstanding Recorded Investment	$ 37	$ 13
Commercial and Industrial [Member] | Prior-Year Accruing [Member] | Commercial Real Estate Portfolio Segment [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	0	0
Pre-modification Outstanding Recorded Investment	$ 0	$ 0
Post-modification Outstanding Recorded Investment	$ 0	$ 0
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	$ 13	$ 16
Post-modification Outstanding Recorded Investment	$ 9	$ 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	$ 13	$ 16
Post-modification Outstanding Recorded Investment	$ 9	$ 12
Real Estate - Residential Mortgage [Member] | Nonperforming [Member] | Consumer Portfolio Segment [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	264	308
Pre-modification Outstanding Recorded Investment	$ 18	$ 18
Post-modification Outstanding Recorded Investment	$ 17	$ 18
Real Estate - Residential Mortgage [Member] | Prior-Year Accruing [Member] | Consumer Portfolio Segment [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	517	484
Pre-modification Outstanding Recorded Investment	$ 37	$ 31
Post-modification Outstanding Recorded Investment	$ 32	$ 31
Home Equity Loans [Member] | Consumer Portfolio Segment [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	1,072	1,025
Pre-modification Outstanding Recorded Investment	$ 60	$ 64
Post-modification Outstanding Recorded Investment	$ 57	$ 57
Home Equity Loans [Member] | Prior-Year Accruing [Member] | Consumer Portfolio Segment [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	1,440	1,276
Pre-modification Outstanding Recorded Investment	$ 85	$ 75
Post-modification Outstanding Recorded Investment	$ 68	$ 59
Consumer Direct Loans [Member] | Nonperforming [Member] | Consumer Portfolio Segment [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	119	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	82	48
Pre-modification Outstanding Recorded Investment	$ 4	$ 3
Post-modification Outstanding Recorded Investment	$ 3	$ 2
Consumer Credit Card [Member] | Nonperforming [Member] | Consumer Portfolio Segment [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	252	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	513	430
Pre-modification Outstanding Recorded Investment	$ 3	$ 1
Post-modification Outstanding Recorded Investment	$ 1	$ 1
Consumer Indirect Loans [Member] | Consumer Portfolio Segment [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	878	825
Pre-modification Outstanding Recorded Investment	$ 16	$ 16
Post-modification Outstanding Recorded Investment	$ 12	$ 13
Consumer Indirect Loans [Member] | Prior-Year Accruing [Member] | Consumer Portfolio Segment [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	501	320
Pre-modification Outstanding Recorded Investment	$ 35	$ 31
Post-modification Outstanding Recorded Investment	$ 22	$ 22