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HMBS Related Obligations - Additional Information (Detail) - USD ($)
$ in Billions
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Residential Loans And Real Estate Owned [Member]    
Debt Instrument [Line Items]    
Unpaid principal balance of assets pledged as collateral to securitization pools $ 9.9  
Carrying value of assets pledged as collateral to securitzation pools $ 10.4  
Home Equity Conversion Mortgage Backed Security Related Obligation [Member]    
Debt Instrument [Line Items]    
Weighted-average stated interest rate 4.09% 4.04%
Home Equity Conversion Mortgage Backed Security Related Obligation [Member] | Weighted Average [Member]    
Debt Instrument [Line Items]    
Weighted-average remaining life in years 3 years 2 months