Asset Quality (Impaired Loans) (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Unpaid Principal Balance - Impaired Financing Receivable [Abstract]
Unpaid principal balance - Impaired loans with an associated allowance	$ 1,739	$ 1,979
Unpaid principal balance - Impaired loans without an associated allowance	1,523	1,429
Unpaid principal balance - Total impaired loans	3,262	3,408
Recorded Investment - Impaired Financing Receivable [Abstract]
Recorded investment - Impaired loans with an associated allowance	1,371	1,662
Recorded investment - Impaired loans without an associated allowance	1,020	930
Recorded investment - Total impaired loans	2,391	2,592
Associated Allowance - Total impaired loans	287	331
Average Recorded Receivable - Impaired Financing Receivable [Abstract]
Average recorded investment - Impaired loans with an associated allowance	1,521	1,752
Average recorded investment - Impaired loans without an associated allowance	972	969
Average recorded investment - Total impaired loans	2,493	2,721
Total commercial lending [Member]
Unpaid Principal Balance - Impaired Financing Receivable [Abstract]
Unpaid principal balance - Impaired loans with an associated allowance	711	742
Unpaid principal balance - Impaired loans without an associated allowance	443	447
Recorded Investment - Impaired Financing Receivable [Abstract]
Recorded investment - Impaired loans with an associated allowance	389	477
Recorded investment - Impaired loans without an associated allowance	329	322
Associated Allowance - Total impaired loans	93	105
Average Recorded Receivable - Impaired Financing Receivable [Abstract]
Average recorded investment - Impaired loans with an associated allowance	435	497
Average recorded investment - Impaired loans without an associated allowance	321	365
Total consumer lending [Member]
Unpaid Principal Balance - Impaired Financing Receivable [Abstract]
Unpaid principal balance - Impaired loans with an associated allowance	1,028	1,237
Unpaid principal balance - Impaired loans without an associated allowance	1,080	982
Recorded Investment - Impaired Financing Receivable [Abstract]
Recorded investment - Impaired loans with an associated allowance	982	1,185
Recorded investment - Impaired loans without an associated allowance	691	608
Associated Allowance - Total impaired loans	194	226
Average Recorded Receivable - Impaired Financing Receivable [Abstract]
Average recorded investment - Impaired loans with an associated allowance	1,086	1,255
Average recorded investment - Impaired loans without an associated allowance	$ 651	$ 604