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Commitments and Contingencies - Additional Information (Detail) - USD ($)
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
Litigation reserve 135,000,000  
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