Asset Quality (Impaired Loans) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Unpaid Principal Balance - Impaired Financing Receivable [Abstract]
Unpaid principal balance - Impaired loans with an associated allowance	$ 1,641	$ 1,979
Unpaid principal balance - Impaired loans without an associated allowance	1,513	1,429
Unpaid principal balance - Total impaired loans	3,154	3,408
Recorded Investment - Impaired Financing Receivable [Abstract]
Recorded investment - Impaired loans with an associated allowance	1,367	1,662
Recorded investment - Impaired loans without an associated allowance	1,004	930
Recorded investment - Total impaired loans	2,371	2,592
Associated Allowance - Total impaired loans	271	331
Average Recorded Receivable - Impaired Financing Receivable [Abstract]
Average recorded investment - Impaired loans with an associated allowance	1,491	1,752
Average recorded investment - Impaired loans without an associated allowance	978	969
Average recorded investment - Total impaired loans	2,469	2,721
Total commercial lending
Unpaid Principal Balance - Impaired Financing Receivable [Abstract]
Unpaid principal balance - Impaired loans with an associated allowance	580	742
Unpaid principal balance - Impaired loans without an associated allowance	494	447
Recorded Investment - Impaired Financing Receivable [Abstract]
Recorded investment - Impaired loans with an associated allowance	353	477
Recorded investment - Impaired loans without an associated allowance	366	322
Associated Allowance - Total impaired loans	76	105
Average Recorded Receivable - Impaired Financing Receivable [Abstract]
Average recorded investment - Impaired loans with an associated allowance	419	497
Average recorded investment - Impaired loans without an associated allowance	330	365
Total consumer lending
Unpaid Principal Balance - Impaired Financing Receivable [Abstract]
Unpaid principal balance - Impaired loans with an associated allowance	1,061	1,237
Unpaid principal balance - Impaired loans without an associated allowance	1,019	982
Recorded Investment - Impaired Financing Receivable [Abstract]
Recorded investment - Impaired loans with an associated allowance	1,014	1,185
Recorded investment - Impaired loans without an associated allowance	638	608
Associated Allowance - Total impaired loans	195	226
Average Recorded Receivable - Impaired Financing Receivable [Abstract]
Average recorded investment - Impaired loans with an associated allowance	1,072	1,255
Average recorded investment - Impaired loans without an associated allowance	$ 648	$ 604