Asset Quality (Impaired Loans) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Unpaid Principal Balance - Impaired Financing Receivable [Abstract]
Unpaid principal balance - Impaired loans with an associated allowance	$ 1,336	$ 1,303
Unpaid principal balance - Impaired loans without an associated allowance	1,372	1,455
Unpaid principal balance - Total impaired loans	2,708	2,758
Recorded Investment - Impaired Financing Receivable [Abstract]
Recorded investment - Impaired loans with an associated allowance	1,191	1,132
Recorded investment - Impaired loans without an associated allowance	867	951
Recorded investment - Total impaired loans	2,058	2,083
Associated Allowance - Total impaired loans	217	209
Average Recorded Receivable - Impaired Financing Receivable [Abstract]
Average recorded investment - Impaired loans with an associated allowance	1,162	1,253
Average recorded investment - Impaired loans without an associated allowance	909	939
Average recorded investment - Total impaired loans	2,071	2,192
Total commercial lending [Member]
Unpaid Principal Balance - Impaired Financing Receivable [Abstract]
Unpaid principal balance - Impaired loans with an associated allowance	481	440
Unpaid principal balance - Impaired loans without an associated allowance	358	413
Recorded Investment - Impaired Financing Receivable [Abstract]
Recorded investment - Impaired loans with an associated allowance	383	315
Recorded investment - Impaired loans without an associated allowance	263	326
Associated Allowance - Total impaired loans	87	73
Average Recorded Receivable - Impaired Financing Receivable [Abstract]
Average recorded investment - Impaired loans with an associated allowance	349	349
Average recorded investment - Impaired loans without an associated allowance	295	294
Total consumer lending [Member]
Unpaid Principal Balance - Impaired Financing Receivable [Abstract]
Unpaid principal balance - Impaired loans with an associated allowance	855	863
Unpaid principal balance - Impaired loans without an associated allowance	1,014	1,042
Recorded Investment - Impaired Financing Receivable [Abstract]
Recorded investment - Impaired loans with an associated allowance	808	817
Recorded investment - Impaired loans without an associated allowance	604	625
Associated Allowance - Total impaired loans	130	136
Average Recorded Receivable - Impaired Financing Receivable [Abstract]
Average recorded investment - Impaired loans with an associated allowance	813	904
Average recorded investment - Impaired loans without an associated allowance	$ 614	$ 645