Asset Quality (Financial Impact and TDRs by Concession Type) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) loans	Dec. 31, 2016 USD ($) loans	Dec. 31, 2015 USD ($) loans
Financing Receivable, Modifications [Line Items]
Number of Loans | loans	12,113	11,405	11,120
Pre-TDR Recorded Investment	$ 541	$ 769	$ 595
Post-TDR Recorded Investment	495	703	520
Principal Forgiveness
Financing Receivable, Modifications [Line Items]
Post-TDR Recorded Investment	18		22
Rate Reduction
Financing Receivable, Modifications [Line Items]
Post-TDR Recorded Investment	153	214	194
Other
Financing Receivable, Modifications [Line Items]
Post-TDR Recorded Investment	$ 324	$ 489	$ 304
Total commercial lending
Financing Receivable, Modifications [Line Items]
Number of Loans | loans	120	143	158
Pre-TDR Recorded Investment	$ 293	$ 524	$ 284
Post-TDR Recorded Investment	252	470	224
Total commercial lending | Principal Forgiveness
Financing Receivable, Modifications [Line Items]
Post-TDR Recorded Investment	18		22
Total commercial lending | Rate Reduction
Financing Receivable, Modifications [Line Items]
Post-TDR Recorded Investment	7	57	4
Total commercial lending | Other
Financing Receivable, Modifications [Line Items]
Post-TDR Recorded Investment	$ 227	$ 413	$ 198
Total consumer lending
Financing Receivable, Modifications [Line Items]
Number of Loans | loans	11,993	11,262	10,962
Pre-TDR Recorded Investment	$ 248	$ 245	$ 311
Post-TDR Recorded Investment	243	233	296
Total consumer lending | Rate Reduction
Financing Receivable, Modifications [Line Items]
Post-TDR Recorded Investment	146	157	190
Total consumer lending | Other
Financing Receivable, Modifications [Line Items]
Post-TDR Recorded Investment	$ 97	$ 76	$ 106