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Details of Repurchase Agreements (Detail) - USD ($)
$ in Thousands
Dec. 31, 2015
Dec. 31, 2014
Debt Outstanding [Line Items]    
Contractual Maturity Over 90 days, amount $ 1,500,000 $ 3,425,000
Contractual Maturity Over 90 days, Weighted Average Interest Rate 1.53%  
Mortgage-Related Securities    
Debt Outstanding [Line Items]    
Contractual Maturity Over 90 days, Amortized Cost $ 766,877  
Contractual Maturity Over 90 days, Fair Value 800,653  
U.S. Treasury and GSE Obligations    
Debt Outstanding [Line Items]    
Contractual Maturity Over 90 days, Amortized Cost 825,844  
Contractual Maturity Over 90 days, Fair Value $ 825,116