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Guarantee Arrangements, Pledged Assets and Repurchase Agreements - Summary of Change in Estimated Repurchase Liability for Loans Sold to GSEs and Other Third Parties (Detail) - USD ($)
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Estimated Repurchase Liability For Loans Sold [Roll Forward]    
Estimate of possible losses in excess of recorded liability $ 325,000,000  
Obligation to Repurchase Receivables Sold [Member]    
Estimated Repurchase Liability For Loans Sold [Roll Forward]    
Balance at beginning of period 10,000,000 $ 12,000,000
Realized losses 0 (1,000,000)
Balance at end of period 10,000,000 $ 11,000,000
Minimum [Member] | Obligation to Repurchase Receivables Sold [Member]    
Estimated Repurchase Liability For Loans Sold [Roll Forward]    
Estimate of possible losses in excess of recorded liability 0  
Maximum [Member] | Obligation to Repurchase Receivables Sold [Member]    
Estimated Repurchase Liability For Loans Sold [Roll Forward]    
Estimate of possible losses in excess of recorded liability $ 25,000,000