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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 6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Estimated Repurchase Liability For Loans Sold [Roll Forward]        
Estimate of possible losses in excess of recorded liability $ 350,000,000   $ 350,000,000  
Obligation to Repurchase Receivables Sold [Member]        
Estimated Repurchase Liability For Loans Sold [Roll Forward]        
Balance at beginning of period 11,000,000 $ 16,000,000 12,000,000 $ 17,000,000
Increase (decrease) in liability recorded through earnings (1,000,000) (3,000,000) (1,000,000) (3,000,000)
Realized losses 1,000,000 0 0 (1,000,000)
Balance at end of period 11,000,000 $ 13,000,000 11,000,000 $ 13,000,000
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   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   $ 25,000,000