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Agency Mortgage-Backed Securities - Portfolio (Detail) - Agency Mortgage Backed Securities - USD ($)
$ in Thousands
Sep. 30, 2015
Dec. 31, 2014
Mortgage-Backed Securities Portfolio:    
Principal outstanding $ 60,733,575 $ 75,982,999
Unamortized premium 4,855,360 5,368,726
Unamortized discount (12,251) (18,200)
Amortized cost 65,576,684 81,333,525
Gross unrealized gains 798,584 982,001
Gross unrealized losses (568,628) (750,270)
Estimated fair value 65,806,640 81,565,256 [1]
Total amortized cost 65,576,684 81,333,525
Fixed Rate    
Mortgage-Backed Securities Portfolio:    
Amortized cost 61,727,096 78,250,313
Gross unrealized gains 668,600 847,615
Gross unrealized losses (551,434) (732,533)
Estimated fair value 61,844,262 78,365,395
Total amortized cost 61,727,096 78,250,313
Adjustable Rate    
Mortgage-Backed Securities Portfolio:    
Amortized cost 3,849,588 3,083,212
Gross unrealized gains 129,984 134,386
Gross unrealized losses (17,194) (17,737)
Estimated fair value 3,962,378 3,199,861
Total amortized cost 3,849,588 3,083,212
Freddie Mac    
Mortgage-Backed Securities Portfolio:    
Principal outstanding 21,598,845 27,906,221
Unamortized premium 1,913,855 1,951,798
Unamortized discount (6,039) (8,985)
Amortized cost 23,506,661 29,849,034
Gross unrealized gains 244,683 313,761
Gross unrealized losses (249,094) (322,094)
Estimated fair value 23,502,250 29,840,701
Total amortized cost 23,506,661 29,849,034
Fannie Mae    
Mortgage-Backed Securities Portfolio:    
Principal outstanding 39,061,904 47,979,778
Unamortized premium 2,909,606 3,396,368
Unamortized discount (6,201) (8,857)
Amortized cost 41,965,309 51,367,289
Gross unrealized gains 547,434 660,230
Gross unrealized losses (316,455) (424,800)
Estimated fair value 42,196,288 51,602,719
Total amortized cost 41,965,309 51,367,289
Ginnie Mae    
Mortgage-Backed Securities Portfolio:    
Principal outstanding 72,826 97,000
Unamortized premium 31,899 20,560
Unamortized discount (11) (358)
Amortized cost 104,714 117,202
Gross unrealized gains 6,467 8,010
Gross unrealized losses (3,079) (3,376)
Estimated fair value 108,102 121,836
Total amortized cost $ 104,714 $ 117,202
[1] Derived from the audited consolidated financial statements at December 31, 2014.