Commitments and Contingencies - Additional Information (Detail) - USD ($)	3 Months Ended
	Sep. 30, 2018	Mar. 31, 2018	Dec. 31, 2017
Loss Contingencies [Line Items]
Maximum credit risk for recourse associated with loans sold under Federal National Mortgage Association Delegated Underwriting and Servicing program	$ 3,300,000,000		$ 3,300,000,000
Increase in litigation reserve		$ 135,000,000
Payment of legal settlement expense	200,000,000
Minimum [Member]
Loss Contingencies [Line Items]
Range of reasonably possible losses	0
Maximum [Member]
Loss Contingencies [Line Items]
Range of reasonably possible losses	$ 50,000,000