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		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 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		$ 1	$ (1)
Net Change in net derivative asset and liability unrealized gains (losses) relating to assets still held at end of period	1		1	(1)
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	276	$ 622	336	979
Net Change in net derivative asset and liability unrealized gains (losses) relating to assets still held at end of period	100	334	100	334
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	279	110	(716)	1,500
Net Change in net derivative asset and liability unrealized gains (losses) relating to assets still held at end of period	$ 311	$ 306	$ (597)	$ 1,700