Commitments and Contingencies - Additional Information (Detail) - USD ($)	Sep. 30, 2021	Dec. 31, 2020
Loss Contingencies [Line Items]
Commitments to extend credit to commercial customers	$ 10,600,000,000	$ 10,400,000,000
Maximum credit risk for recourse associated with loans sold under Federal National Mortgage Association Delegated Underwriting and Servicing program	3,900,000,000	$ 4,000,000,000.0
Minimum [Member]
Loss Contingencies [Line Items]
Range of reasonably possible losses	0
Maximum [Member]
Loss Contingencies [Line Items]
Range of reasonably possible losses	$ 25,000,000