Fair Values of Assets and Liabilities - Changes in Fair Value for All Assets and Liabilities Measured at Fair Value on Recurring Basis Using Significant Unobservable Inputs (Level 3) (Parenthetical) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Other Noninterest Income [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Net gains and (losses) on net derivative assets and liabilities included in net income	$ 1	$ (69)	$ 2	$ (70)
Net Change in net derivative asset and liability unrealized gains (losses) relating to assets still held at end of period	1	(69)	2	(70)
Mortgage Banking Revenue [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Net gains and (losses) on net derivative assets and liabilities included in net income	208	508	544	1,500
Net Change in net derivative asset and liability unrealized gains (losses) relating to assets still held at end of period	57	291	57	291
Commercial Products Revenue [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Net gains and (losses) on net derivative assets and liabilities included in net income	(434)	(228)	(1,200)	1,300
Net Change in net derivative asset and liability unrealized gains (losses) relating to assets still held at end of period	$ (261)	$ 6	$ (820)	$ 1,700