Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Mar. 31, 2024	Sep. 30, 2024	Dec. 31, 2023
Loss Contingencies [Line Items]
Commitments to extend credit to commercial customers		$ 12,700	$ 12,300
Maximum credit risk for recourse associated with loans sold under Federal National Mortgage Association Delegated Underwriting and Servicing program		3,900	$ 3,900
Estimate assessed losses	$ 197	34
Minimum
Loss Contingencies [Line Items]
Range of reasonably possible losses		0
Maximum
Loss Contingencies [Line Items]
Range of reasonably possible losses		$ 25