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Component of Agency Mortgage-Backed Securities Portfolio by Issuing Agency Concentration (Detail) - USD ($)
$ in Thousands
Mar. 31, 2016
Dec. 31, 2015
Mortgage-Backed Securities Portfolio:    
Estimated Fair Value $ 67,255,533 $ 67,233,494
Agency Mortgage Backed Securities    
Mortgage-Backed Securities Portfolio:    
Estimated Fair Value 65,439,824 65,718,224 [1]
Agency Mortgage Backed Securities | Fannie Mae    
Mortgage-Backed Securities Portfolio:    
Estimated Fair Value 41,990,479 42,647,075
Agency Mortgage Backed Securities | Freddie Mac    
Mortgage-Backed Securities Portfolio:    
Estimated Fair Value 23,341,571 22,960,595
Agency Mortgage Backed Securities | Ginnie Mae    
Mortgage-Backed Securities Portfolio:    
Estimated Fair Value $ 107,774 $ 110,554
[1] Derived from the audited consolidated financial statements at December 31, 2015.