Asset Quality (Financial Impact and TDRs by Concession Type) (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017 USD ($) loans	Sep. 30, 2016 USD ($) loans	Sep. 30, 2017 USD ($) loans	Sep. 30, 2016 USD ($) loans
Financing Receivable, Modifications [Line Items]
Number of Loans | loans	2,990	2,837	8,946	8,544
Pre-TDR Recorded Investment	$ 96	$ 170	$ 435	$ 667
Post-TDR Recorded Investment	95	156	393	609
Principal Forgiveness [Member]
Financing Receivable, Modifications [Line Items]
Post-TDR Recorded Investment	14		18
Rate Reduction [Member]
Financing Receivable, Modifications [Line Items]
Post-TDR Recorded Investment	36	38	122	172
Other [Member]
Financing Receivable, Modifications [Line Items]
Post-TDR Recorded Investment	$ 45	$ 118	$ 253	$ 437
Total commercial lending [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | loans	25	37	107	109
Pre-TDR Recorded Investment	$ 44	$ 108	$ 256	$ 480
Post-TDR Recorded Investment	44	97	215	432
Total commercial lending [Member] | Principal Forgiveness [Member]
Financing Receivable, Modifications [Line Items]
Post-TDR Recorded Investment	14		18
Total commercial lending [Member] | Rate Reduction [Member]
Financing Receivable, Modifications [Line Items]
Post-TDR Recorded Investment	0	1	6	53
Total commercial lending [Member] | Other [Member]
Financing Receivable, Modifications [Line Items]
Post-TDR Recorded Investment	$ 30	$ 96	$ 191	$ 379
Total consumer lending [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | loans	2,965	2,800	8,839	8,435
Pre-TDR Recorded Investment	$ 52	$ 62	$ 179	$ 187
Post-TDR Recorded Investment	51	59	178	177
Total consumer lending [Member] | Rate Reduction [Member]
Financing Receivable, Modifications [Line Items]
Post-TDR Recorded Investment	36	37	116	119
Total consumer lending [Member] | Other [Member]
Financing Receivable, Modifications [Line Items]
Post-TDR Recorded Investment	$ 15	$ 22	$ 62	$ 58