Loans and Allowance for Credit Losses - Summary of Loans Modified as TDRs (Detail) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019 USD ($) SecurityLoan	Sep. 30, 2018 USD ($) SecurityLoan	Sep. 30, 2019 USD ($) SecurityLoan	Sep. 30, 2018 USD ($) SecurityLoan
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	11,802	11,747	35,680	33,643
Pre-Modification Outstanding Loan Balance	$ 433	$ 460	$ 1,193	$ 1,264
Post-Modification Outstanding Loan Balance	$ 406	$ 446	$ 1,138	$ 1,229
Commercial [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	886	700	2,622	2,047
Pre-Modification Outstanding Loan Balance	$ 116	$ 42	$ 242	$ 255
Post-Modification Outstanding Loan Balance	$ 100	$ 33	$ 215	$ 234
Commercial Real Estate [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	32	38	76	97
Pre-Modification Outstanding Loan Balance	$ 23	$ 123	$ 95	$ 154
Post-Modification Outstanding Loan Balance	$ 23	$ 125	$ 93	$ 155
Residential Mortgages [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	117	144	318	397
Pre-Modification Outstanding Loan Balance	$ 17	$ 19	$ 43	$ 56
Post-Modification Outstanding Loan Balance	$ 15	$ 17	$ 41	$ 53
Other Retail [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	814	763	2,029	1,857
Pre-Modification Outstanding Loan Balance	$ 20	$ 17	$ 44	$ 45
Post-Modification Outstanding Loan Balance	$ 19	$ 16	$ 42	$ 43
Credit Card [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	8,429	8,450	26,018	24,457
Pre-Modification Outstanding Loan Balance	$ 46	$ 42	$ 140	$ 122
Post-Modification Outstanding Loan Balance	$ 46	$ 43	$ 141	$ 124
Covered Loans [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan		3		3
Pre-Modification Outstanding Loan Balance		$ 1		$ 1
Post-Modification Outstanding Loan Balance		$ 1		$ 1
Total Loans, Excluding Loans Purchased from GNMA Mortgage Pools [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	10,278	10,098	31,063	28,858
Pre-Modification Outstanding Loan Balance	$ 222	$ 244	$ 564	$ 633
Post-Modification Outstanding Loan Balance	$ 203	$ 235	$ 532	$ 610
Government National Mortgage Association [Member]
Financing Receivable, Modifications [Line Items]
Number of Loans | SecurityLoan	1,524	1,649	4,617	4,785
Pre-Modification Outstanding Loan Balance	$ 211	$ 216	$ 629	$ 631
Post-Modification Outstanding Loan Balance	$ 203	$ 211	$ 606	$ 619