Commitments and Contingencies - Additional Information (Detail) - USD ($)	Sep. 30, 2015	Dec. 31, 2014
Commitments and Contingencies Disclosure [Abstract]
Maximum credit risk for recourse associated with loans sold under Federal National Mortgage Association Delegated Underwriting and Servicing program	$ 2,500,000,000	$ 2,400,000,000
Range of reasonably possible losses, minimum	0
Range of reasonably possible losses, maximum	$ 40,000,000