Asset Quality - Breakdown of Nonperforming TDRs by Loans Category (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) SecurityLoan	Dec. 31, 2016 USD ($) SecurityLoan
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	5,184	4,515
Pre-modification Outstanding Recorded Investment	$ 398	$ 389
Post-modification Outstanding Recorded Investment	$ 317	$ 280
Prior-Year Accruing [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	2,562	2,535
Pre-modification Outstanding Recorded Investment	$ 171	$ 196
Post-modification Outstanding Recorded Investment	$ 128	$ 139
Nonperforming [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	2,622	1,980
Pre-modification Outstanding Recorded Investment	$ 227	$ 193
Post-modification Outstanding Recorded Investment	$ 189	$ 141
Commercial Loans [Member] | Prior-Year Accruing [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	4	5
Pre-modification Outstanding Recorded Investment	$ 30	$ 30
Post-modification Outstanding Recorded Investment	$ 13	$ 16
Commercial Loans [Member] | Commercial, Financial and Agricultural [Member] | Prior-Year Accruing [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	4	5
Pre-modification Outstanding Recorded Investment	$ 30	$ 30
Post-modification Outstanding Recorded Investment	$ 13	$ 16
Commercial Loans [Member] | Nonperforming [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	28	25
Pre-modification Outstanding Recorded Investment	$ 125	$ 93
Post-modification Outstanding Recorded Investment	$ 98	$ 51
Commercial Loans [Member] | Nonperforming [Member] | Commercial, Financial and Agricultural [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	20	18
Pre-modification Outstanding Recorded Investment	$ 109	$ 91
Post-modification Outstanding Recorded Investment	$ 86	$ 50
Commercial Loans [Member] | Nonperforming [Member] | Commercial Real Estate: Commercial Mortgage [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	8	7
Pre-modification Outstanding Recorded Investment	$ 16	$ 2
Post-modification Outstanding Recorded Investment	$ 12	$ 1
Commercial Loans [Member] | Nonperforming [Member] | Commercial Real Estate Loans [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	8	7
Pre-modification Outstanding Recorded Investment	$ 16	$ 2
Post-modification Outstanding Recorded Investment	$ 12	$ 1
Consumer Loans [Member] | Prior-Year Accruing [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	2,558	2,530
Pre-modification Outstanding Recorded Investment	$ 141	$ 166
Post-modification Outstanding Recorded Investment	$ 115	$ 123
Consumer Loans [Member] | Real Estate - Residential Mortgage [Member] | Prior-Year Accruing [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	484	477
Pre-modification Outstanding Recorded Investment	$ 31	$ 35
Post-modification Outstanding Recorded Investment	$ 31	$ 35
Consumer Loans [Member] | Home Equity Loan [Member] | Prior-Year Accruing [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	1,276	1,231
Pre-modification Outstanding Recorded Investment	$ 75	$ 70
Post-modification Outstanding Recorded Investment	$ 59	$ 57
Consumer Loans [Member] | Consumer Direct Loans [Member] | Prior-Year Accruing [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	48	35
Pre-modification Outstanding Recorded Investment	$ 3	$ 2
Post-modification Outstanding Recorded Investment	$ 2	$ 2
Consumer Loans [Member] | Consumer Credit Card [Member] | Prior-Year Accruing [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	430	410
Pre-modification Outstanding Recorded Investment	$ 1	$ 3
Post-modification Outstanding Recorded Investment	$ 1	$ 1
Consumer Loans [Member] | Consumer Indirect Loans [Member] | Prior-Year Accruing [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	320	377
Pre-modification Outstanding Recorded Investment	$ 31	$ 56
Post-modification Outstanding Recorded Investment	$ 22	$ 28
Consumer Loans [Member] | Nonperforming [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	2,594	1,955
Pre-modification Outstanding Recorded Investment	$ 102	$ 100
Post-modification Outstanding Recorded Investment	$ 91	$ 90
Consumer Loans [Member] | Nonperforming [Member] | Real Estate - Residential Mortgage [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	308	264
Pre-modification Outstanding Recorded Investment	$ 18	$ 16
Post-modification Outstanding Recorded Investment	$ 18	$ 16
Consumer Loans [Member] | Nonperforming [Member] | Home Equity Loan [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	1,025
Pre-modification Outstanding Recorded Investment	$ 64
Post-modification Outstanding Recorded Investment	$ 57
Consumer Loans [Member] | Nonperforming [Member] | Home Equity Loan [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan		1,199
Pre-modification Outstanding Recorded Investment		$ 77
Post-modification Outstanding Recorded Investment		$ 69
Consumer Loans [Member] | Nonperforming [Member] | Consumer Direct Loans [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	114	32
Pre-modification Outstanding Recorded Investment	$ 2	$ 1
Post-modification Outstanding Recorded Investment	$ 2	$ 0
Consumer Loans [Member] | Nonperforming [Member] | Consumer Credit Card [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	322	336
Pre-modification Outstanding Recorded Investment	$ 2	$ 2
Post-modification Outstanding Recorded Investment	$ 1	$ 2
Consumer Loans [Member] | Nonperforming [Member] | Consumer Indirect Loans [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	825
Pre-modification Outstanding Recorded Investment	$ 16
Post-modification Outstanding Recorded Investment	$ 13
Consumer Loans [Member] | Nonperforming [Member] | Consumer Indirect Loans [Member] | Nonperforming [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan		124
Pre-modification Outstanding Recorded Investment		$ 4
Post-modification Outstanding Recorded Investment		$ 3