Asset Quality (Impaired Loans) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Unpaid Principal Balance - Impaired Financing Receivable [Abstract]
Unpaid principal balance - Impaired loans with an associated allowance	$ 1,517	$ 1,641
Unpaid principal balance - Impaired loans without an associated allowance	1,585	1,513
Unpaid principal balance - Total impaired loans	3,102	3,154
Recorded Investment - Impaired Financing Receivable [Abstract]
Recorded investment - Impaired loans with an associated allowance	1,286	1,367
Recorded investment - Impaired loans without an associated allowance	1,019	1,004
Recorded investment - Total impaired loans	2,305	2,371
Associated Allowance - Total impaired loans	253	271
Average Recorded Receivable - Impaired Financing Receivable [Abstract]
Average recorded investment - Impaired loans with an associated allowance	1,327	1,491
Average recorded investment - Impaired loans without an associated allowance	1,011	978
Average recorded investment - Total impaired loans	2,338	2,469
Total commercial lending [Member]
Unpaid Principal Balance - Impaired Financing Receivable [Abstract]
Unpaid principal balance - Impaired loans with an associated allowance	537	580
Unpaid principal balance - Impaired loans without an associated allowance	427	494
Recorded Investment - Impaired Financing Receivable [Abstract]
Recorded investment - Impaired loans with an associated allowance	371	353
Recorded investment - Impaired loans without an associated allowance	326	366
Associated Allowance - Total impaired loans	101	76
Average Recorded Receivable - Impaired Financing Receivable [Abstract]
Average recorded investment - Impaired loans with an associated allowance	363	419
Average recorded investment - Impaired loans without an associated allowance	345	330
Total consumer lending [Member]
Unpaid Principal Balance - Impaired Financing Receivable [Abstract]
Unpaid principal balance - Impaired loans with an associated allowance	980	1,061
Unpaid principal balance - Impaired loans without an associated allowance	1,158	1,019
Recorded Investment - Impaired Financing Receivable [Abstract]
Recorded investment - Impaired loans with an associated allowance	915	1,014
Recorded investment - Impaired loans without an associated allowance	693	638
Associated Allowance - Total impaired loans	152	195
Average Recorded Receivable - Impaired Financing Receivable [Abstract]
Average recorded investment - Impaired loans with an associated allowance	964	1,072
Average recorded investment - Impaired loans without an associated allowance	$ 666	$ 648