Asset Quality - Aging Analysis of Past Due and Current Loans (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	$ 87,562	$ 84,716
Days Past Due	1,100	951
Non-performing Loans	645	503
Purchased Credit Impaired	606	738
Total loans	89,268	86,405
Commercial Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	65,072	61,904
Days Past Due	639	472
Non-performing Loans	337	191
Purchased Credit Impaired	261	357
Total loans	65,972	62,733
Consumer Portfolio Segment [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	22,490	22,812
Days Past Due	461	479
Non-performing Loans	308	312
Purchased Credit Impaired	345	381
Total loans	23,296	23,672
Commercial and Industrial [Member] | Commercial Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	44,561	41,444
Days Past Due	408	332
Non-performing Loans	227	153
Purchased Credit Impaired	54	83
Total loans	45,023	41,859
Commercial Real Estate: Commercial Mortgage [Member] | Commercial Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	14,347	13,750
Days Past Due	165	90
Non-performing Loans	98	30
Purchased Credit Impaired	204	248
Total loans	14,716	14,088
Commercial Real Estate: Construction [Member] | Commercial Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	1,715	1,919
Days Past Due	45	15
Non-performing Loans	2	2
Purchased Credit Impaired	3	26
Total loans	1,763	1,960
Commercial Real Estate Loans [Member] | Commercial Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	16,062	15,669
Days Past Due	210	105
Non-performing Loans	100	32
Purchased Credit Impaired	207	274
Total loans	16,479	16,048
Commercial Lease Financing [Member] | Commercial Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	4,449	4,791
Days Past Due	21	35
Non-performing Loans	10	6
Purchased Credit Impaired	0	0
Total loans	4,470	4,826
Real Estate - Residential Mortgage [Member] | Consumer Portfolio Segment [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	5,094	5,043
Days Past Due	80	85
Non-performing Loans	62	58
Purchased Credit Impaired	323	355
Total loans	5,497	5,483
Home Equity Loans [Member] | Consumer Portfolio Segment [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	11,044	11,721
Days Past Due	276	285
Non-performing Loans	221	229
Purchased Credit Impaired	19	22
Total loans	11,339	12,028
Consumer Direct Loans [Member] | Consumer Portfolio Segment [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	1,779	1,768
Days Past Due	25	22
Non-performing Loans	4	4
Purchased Credit Impaired	3	4
Total loans	1,807	1,794
Consumer Credit Card Financing Receivable [Member] | Consumer Portfolio Segment [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	1,075	1,081
Days Past Due	23	25
Non-performing Loans	2	2
Purchased Credit Impaired	0	0
Total loans	1,098	1,106
Consumer Indirect Loans [Member] | Consumer Portfolio Segment [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	3,498	3,199
Days Past Due	57	62
Non-performing Loans	19	19
Purchased Credit Impaired	0	0
Total loans	3,555	3,261
30-59 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Days Past Due	220	261
30-59 Days Past Due [Member] | Commercial Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Days Past Due	127	164
30-59 Days Past Due [Member] | Consumer Portfolio Segment [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Days Past Due	93	97
30-59 Days Past Due [Member] | Commercial and Industrial [Member] | Commercial Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Days Past Due	74	111
30-59 Days Past Due [Member] | Commercial Real Estate: Commercial Mortgage [Member] | Commercial Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Days Past Due	24	26
30-59 Days Past Due [Member] | Commercial Real Estate: Construction [Member] | Commercial Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Days Past Due	20	4
30-59 Days Past Due [Member] | Commercial Real Estate Loans [Member] | Commercial Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Days Past Due	44	30
30-59 Days Past Due [Member] | Commercial Lease Financing [Member] | Commercial Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Days Past Due	9	23
30-59 Days Past Due [Member] | Real Estate - Residential Mortgage [Member] | Consumer Portfolio Segment [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Days Past Due	13	16
30-59 Days Past Due [Member] | Home Equity Loans [Member] | Consumer Portfolio Segment [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Days Past Due	36	32
30-59 Days Past Due [Member] | Consumer Direct Loans [Member] | Consumer Portfolio Segment [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Days Past Due	9	9
30-59 Days Past Due [Member] | Consumer Credit Card Financing Receivable [Member] | Consumer Portfolio Segment [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Days Past Due	6	7
30-59 Days Past Due [Member] | Consumer Indirect Loans [Member] | Consumer Portfolio Segment [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Days Past Due	29	33
60-89 Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Days Past Due	148	98
60-89 Days Past Due [Member] | Commercial Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Days Past Due	118	60
60-89 Days Past Due [Member] | Consumer Portfolio Segment [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Days Past Due	30	38
60-89 Days Past Due [Member] | Commercial and Industrial [Member] | Commercial Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Days Past Due	83	34
60-89 Days Past Due [Member] | Commercial Real Estate: Commercial Mortgage [Member] | Commercial Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Days Past Due	20	13
60-89 Days Past Due [Member] | Commercial Real Estate: Construction [Member] | Commercial Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Days Past Due	14	9
60-89 Days Past Due [Member] | Commercial Real Estate Loans [Member] | Commercial Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Days Past Due	34	22
60-89 Days Past Due [Member] | Commercial Lease Financing [Member] | Commercial Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Days Past Due	1	4
60-89 Days Past Due [Member] | Real Estate - Residential Mortgage [Member] | Consumer Portfolio Segment [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Days Past Due	3	7
60-89 Days Past Due [Member] | Home Equity Loans [Member] | Consumer Portfolio Segment [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Days Past Due	11	15
60-89 Days Past Due [Member] | Consumer Direct Loans [Member] | Consumer Portfolio Segment [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Days Past Due	5	4
60-89 Days Past Due [Member] | Consumer Credit Card Financing Receivable [Member] | Consumer Portfolio Segment [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Days Past Due	5	5
60-89 Days Past Due [Member] | Consumer Indirect Loans [Member] | Consumer Portfolio Segment [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Days Past Due	6	7
90 and Greater Days Past Due [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Days Past Due	87	89
90 and Greater Days Past Due [Member] | Commercial Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Days Past Due	57	57
90 and Greater Days Past Due [Member] | Consumer Portfolio Segment [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Days Past Due	30	32
90 and Greater Days Past Due [Member] | Commercial and Industrial [Member] | Commercial Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Days Past Due	24	34
90 and Greater Days Past Due [Member] | Commercial Real Estate: Commercial Mortgage [Member] | Commercial Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Days Past Due	23	21
90 and Greater Days Past Due [Member] | Commercial Real Estate: Construction [Member] | Commercial Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Days Past Due	9	0
90 and Greater Days Past Due [Member] | Commercial Real Estate Loans [Member] | Commercial Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Days Past Due	32	21
90 and Greater Days Past Due [Member] | Commercial Lease Financing [Member] | Commercial Loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Days Past Due	1	2
90 and Greater Days Past Due [Member] | Real Estate - Residential Mortgage [Member] | Consumer Portfolio Segment [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Days Past Due	2	4
90 and Greater Days Past Due [Member] | Home Equity Loans [Member] | Consumer Portfolio Segment [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Days Past Due	8	9
90 and Greater Days Past Due [Member] | Consumer Direct Loans [Member] | Consumer Portfolio Segment [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Days Past Due	7	5
90 and Greater Days Past Due [Member] | Consumer Credit Card Financing Receivable [Member] | Consumer Portfolio Segment [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Days Past Due	10	11
90 and Greater Days Past Due [Member] | Consumer Indirect Loans [Member] | Consumer Portfolio Segment [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Days Past Due	$ 3	$ 3