Asset Quality (Financial Impact and TDRs by Concession Type) (Details) $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($) loans	Mar. 31, 2017 USD ($) loans
Financing Receivable, Modifications [Line Items]
Number of Loans | loans	3,011	2,948
Pre-TDR Recorded Investment	$ 59	$ 108
Post-TDR Recorded Investment	54	83
Principal Forgiveness [Member]
Financing Receivable, Modifications [Line Items]
Post-TDR Recorded Investment		4
Rate Reduction [Member]
Financing Receivable, Modifications [Line Items]
Post-TDR Recorded Investment	31	43
Other [Member]
Financing Receivable, Modifications [Line Items]
Post-TDR Recorded Investment	$ 23	$ 36
Total commercial lending [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | loans	32	49
Pre-TDR Recorded Investment	$ 10	$ 35
Post-TDR Recorded Investment	8	15
Total commercial lending [Member] | Principal Forgiveness [Member]
Financing Receivable, Modifications [Line Items]
Post-TDR Recorded Investment		4
Total commercial lending [Member] | Rate Reduction [Member]
Financing Receivable, Modifications [Line Items]
Post-TDR Recorded Investment	1	6
Total commercial lending [Member] | Other [Member]
Financing Receivable, Modifications [Line Items]
Post-TDR Recorded Investment	$ 7	$ 5
Total consumer lending [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | loans	2,979	2,899
Pre-TDR Recorded Investment	$ 49	$ 73
Post-TDR Recorded Investment	46	68
Total consumer lending [Member] | Rate Reduction [Member]
Financing Receivable, Modifications [Line Items]
Post-TDR Recorded Investment	30	37
Total consumer lending [Member] | Other [Member]
Financing Receivable, Modifications [Line Items]
Post-TDR Recorded Investment	$ 16	$ 31