Loans and Allowance for Credit Losses - Summary of Loans Modified as TDRs (Detail) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($) SecurityLoan	Sep. 30, 2017 USD ($) SecurityLoan	Sep. 30, 2018 USD ($) SecurityLoan	Sep. 30, 2017 USD ($) SecurityLoan
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	11,747	12,184	33,643	37,040
Pre-Modification Outstanding Loan Balance	$ 460	$ 319	$ 1,264	$ 1,254
Post-Modification Outstanding Loan Balance	$ 446	$ 300	$ 1,229	$ 1,190
Commercial [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	700	616	2,047	2,117
Pre-Modification Outstanding Loan Balance	$ 42	$ 40	$ 255	$ 239
Post-Modification Outstanding Loan Balance	$ 33	$ 27	$ 234	$ 195
Commercial Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	38	29	97	93
Pre-Modification Outstanding Loan Balance	$ 123	$ 18	$ 154	$ 56
Post-Modification Outstanding Loan Balance	$ 125	$ 16	$ 155	$ 55
Residential Mortgages [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	144	141	397	641
Pre-Modification Outstanding Loan Balance	$ 19	$ 15	$ 56	$ 72
Post-Modification Outstanding Loan Balance	$ 17	$ 16	$ 53	$ 73
Other Retail [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	763	1,949	1,857	3,210
Pre-Modification Outstanding Loan Balance	$ 17	$ 39	$ 45	$ 65
Post-Modification Outstanding Loan Balance	$ 16	$ 32	$ 43	$ 55
Credit Card [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	8,450	8,106	24,457	25,657
Pre-Modification Outstanding Loan Balance	$ 42	$ 38	$ 122	$ 123
Post-Modification Outstanding Loan Balance	$ 43	$ 38	$ 124	$ 124
Total Loans, Excluding GNMA and Covered Loans [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	10,095	10,841	28,855	31,718
Pre-Modification Outstanding Loan Balance	$ 243	$ 150	$ 632	$ 555
Post-Modification Outstanding Loan Balance	$ 234	$ 129	$ 609	$ 502
Covered Loans [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	3	3	3	10
Pre-Modification Outstanding Loan Balance	$ 1		$ 1	$ 2
Post-Modification Outstanding Loan Balance	$ 1		$ 1	$ 2
Government National Mortgage Association [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	1,649	1,340	4,785	5,312
Pre-Modification Outstanding Loan Balance	$ 216	$ 169	$ 631	$ 697
Post-Modification Outstanding Loan Balance	$ 211	$ 171	$ 619	$ 686