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Mortgage-Backed Securities - Summary (Detail) - Residential Mortgage-Backed Securities [Member] - USD ($)
$ in Thousands
Mar. 31, 2016
Dec. 31, 2015
Investment Holdings [Line Items]    
Principal or Notional Value $ 20,078,557 $ 22,845,276
Total Premium 768,970 868,410
Total Discount (1,768,301) (1,814,615)
Amortized Cost 9,445,410 9,465,640
Fair Value 10,241,539 10,190,665
Gross Unrealized Gains 879,752 867,512
Gross Unrealized Losses (83,623) (142,490)
Net Unrealized Gain/(Loss) 796,129 725,022
Non-Agency RMBS - Senior interest-only [Member]    
Investment Holdings [Line Items]    
Principal or Notional Value 5,343,042 5,426,029
Total Premium 269,239 268,515
Amortized Cost 269,239 268,515
Fair Value 241,102 234,171
Gross Unrealized Gains 20,674 18,113
Gross Unrealized Losses (48,811) (52,457)
Net Unrealized Gain/(Loss) (28,137) (34,344)
Non-Agency RMBS - Subordinated [Member]    
Investment Holdings [Line Items]    
Principal or Notional Value 735,699 762,466
Total Premium 22,055 23,635
Total Discount (248,773) (258,128)
Amortized Cost 508,981 527,975
Fair Value 582,975 604,295
Gross Unrealized Gains 83,670 83,896
Gross Unrealized Losses (9,676) (7,577)
Net Unrealized Gain/(Loss) 73,994 76,319
Non-Agency RMBS - Subordinated interest-only [Member]    
Investment Holdings [Line Items]    
Principal or Notional Value 279,620 284,931
Total Premium 14,918 15,226
Amortized Cost 14,918 15,226
Fair Value 9,835 11,254
Gross Unrealized Gains 11 62
Gross Unrealized Losses (5,094) (4,035)
Net Unrealized Gain/(Loss) (5,083) (3,973)
Agency MBS - Residential [Member]    
Investment Holdings [Line Items]    
Principal or Notional Value 5,063,355 5,045,418
Total Premium 248,624 255,837
Amortized Cost 5,311,979 5,301,255
Fair Value 5,356,772 5,267,848
Gross Unrealized Gains 46,678 18,593
Gross Unrealized Losses (1,885) (52,001)
Net Unrealized Gain/(Loss) 44,793 (33,408)
Agency MBS - Commercial [Member]    
Investment Holdings [Line Items]    
Principal or Notional Value 1,072,937 952,091
Total Premium 27,991 24,815
Total Discount (3,254) (3,170)
Amortized Cost 1,097,674 973,736
Fair Value 1,125,517 973,787
Gross Unrealized Gains 29,868 8,052
Gross Unrealized Losses (2,025) (8,001)
Net Unrealized Gain/(Loss) 27,843 51
Agency MBS - Interest-only [Member]    
Investment Holdings [Line Items]    
Principal or Notional Value 4,011,154 6,722,472
Total Premium 185,830 280,073
Amortized Cost 185,830 280,073
Fair Value 179,000 273,189
Gross Unrealized Gains 2,391 2,756
Gross Unrealized Losses (9,221) (9,640)
Net Unrealized Gain/(Loss) (6,830) (6,884)
Non-Agency RMBS - Senior [Member]    
Investment Holdings [Line Items]    
Principal or Notional Value 3,572,750 3,651,869
Total Premium   309
Total Discount   (1,553,317)
Amortized Cost 2,056,789 2,098,860
Fair Value 2,746,338 2,826,121
Gross Unrealized Gains 696,460 736,040
Gross Unrealized Losses $ (6,911) (8,779)
Net Unrealized Gain/(Loss)   $ 727,261