Asset Quality (Impaired Loans) (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Unpaid Principal Balance - Impaired Financing Receivable [Abstract]
Unpaid principal balance - Impaired loans with an associated allowance	$ 1,427	$ 1,641
Unpaid principal balance - Impaired loans without an associated allowance	1,365	1,513
Unpaid principal balance - Total impaired loans	2,792	3,154
Recorded Investment - Impaired Financing Receivable [Abstract]
Recorded investment - Impaired loans with an associated allowance	1,212	1,367
Recorded investment - Impaired loans without an associated allowance	880	1,004
Recorded investment - Total impaired loans	2,092	2,371
Associated Allowance - Total impaired loans	210	271
Average Recorded Receivable - Impaired Financing Receivable [Abstract]
Average recorded investment - Impaired loans with an associated allowance	1,282	1,491
Average recorded investment - Impaired loans without an associated allowance	937	978
Average recorded investment - Total impaired loans	2,219	2,469
Total commercial lending [Member]
Unpaid Principal Balance - Impaired Financing Receivable [Abstract]
Unpaid principal balance - Impaired loans with an associated allowance	494	580
Unpaid principal balance - Impaired loans without an associated allowance	326	494
Recorded Investment - Impaired Financing Receivable [Abstract]
Recorded investment - Impaired loans with an associated allowance	333	353
Recorded investment - Impaired loans without an associated allowance	262	366
Associated Allowance - Total impaired loans	65	76
Average Recorded Receivable - Impaired Financing Receivable [Abstract]
Average recorded investment - Impaired loans with an associated allowance	357	419
Average recorded investment - Impaired loans without an associated allowance	287	330
Total consumer lending [Member]
Unpaid Principal Balance - Impaired Financing Receivable [Abstract]
Unpaid principal balance - Impaired loans with an associated allowance	933	1,061
Unpaid principal balance - Impaired loans without an associated allowance	1,039	1,019
Recorded Investment - Impaired Financing Receivable [Abstract]
Recorded investment - Impaired loans with an associated allowance	879	1,014
Recorded investment - Impaired loans without an associated allowance	618	638
Associated Allowance - Total impaired loans	145	195
Average Recorded Receivable - Impaired Financing Receivable [Abstract]
Average recorded investment - Impaired loans with an associated allowance	925	1,072
Average recorded investment - Impaired loans without an associated allowance	$ 650	$ 648