Asset Quality (Impaired Loans) (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Unpaid Principal Balance - Impaired Financing Receivable [Abstract]
Unpaid principal balance - Impaired loans with an associated allowance	$ 1,485	$ 1,641
Unpaid principal balance - Impaired loans without an associated allowance	1,325	1,513
Unpaid principal balance - Total impaired loans	2,810	3,154
Recorded Investment - Impaired Financing Receivable [Abstract]
Recorded investment - Impaired loans with an associated allowance	1,264	1,367
Recorded investment - Impaired loans without an associated allowance	844	1,004
Recorded investment - Total impaired loans	2,108	2,371
Associated Allowance - Total impaired loans	236	271
Average Recorded Receivable - Impaired Financing Receivable [Abstract]
Average recorded investment - Impaired loans with an associated allowance	1,306	1,491
Average recorded investment - Impaired loans without an associated allowance	955	978
Average recorded investment - Total impaired loans	2,261	2,469
Total commercial lending [Member]
Unpaid Principal Balance - Impaired Financing Receivable [Abstract]
Unpaid principal balance - Impaired loans with an associated allowance	541	580
Unpaid principal balance - Impaired loans without an associated allowance	236	494
Recorded Investment - Impaired Financing Receivable [Abstract]
Recorded investment - Impaired loans with an associated allowance	370	353
Recorded investment - Impaired loans without an associated allowance	194	366
Associated Allowance - Total impaired loans	89	76
Average Recorded Receivable - Impaired Financing Receivable [Abstract]
Average recorded investment - Impaired loans with an associated allowance	365	419
Average recorded investment - Impaired loans without an associated allowance	295	330
Total consumer lending [Member]
Unpaid Principal Balance - Impaired Financing Receivable [Abstract]
Unpaid principal balance - Impaired loans with an associated allowance	944	1,061
Unpaid principal balance - Impaired loans without an associated allowance	1,089	1,019
Recorded Investment - Impaired Financing Receivable [Abstract]
Recorded investment - Impaired loans with an associated allowance	894	1,014
Recorded investment - Impaired loans without an associated allowance	650	638
Associated Allowance - Total impaired loans	147	195
Average Recorded Receivable - Impaired Financing Receivable [Abstract]
Average recorded investment - Impaired loans with an associated allowance	941	1,072
Average recorded investment - Impaired loans without an associated allowance	$ 660	$ 648