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Mortgage-Backed Securities - Summary (Detail) - Residential Mortgage-Backed Securities - USD ($)
$ in Thousands
Jun. 30, 2016
Dec. 31, 2015
Investment Holdings [Line Items]    
Principal or Notional Value $ 17,223,903 $ 22,845,276
Total Premium 655,942 868,410
Total Discount (1,742,225) (1,814,615)
Amortized Cost 7,092,105 9,465,640
Fair Value 7,972,983 10,190,665
Gross Unrealized Gains 945,058 867,512
Gross Unrealized Losses (64,180) (142,490)
Net Unrealized Gain/(Loss) 880,878 725,022
Non-Agency RMBS - Senior interest-only    
Investment Holdings [Line Items]    
Principal or Notional Value 5,693,428 5,426,029
Total Premium 295,094 268,515
Amortized Cost 295,094 268,515
Fair Value 280,868 234,171
Gross Unrealized Gains 28,130 18,113
Gross Unrealized Losses (42,356) (52,457)
Net Unrealized Gain/(Loss) (14,226) (34,344)
Non-Agency RMBS - Subordinated    
Investment Holdings [Line Items]    
Principal or Notional Value 718,917 762,466
Total Premium 21,200 23,635
Total Discount (235,506) (258,128)
Amortized Cost 504,613 527,975
Fair Value 577,152 604,295
Gross Unrealized Gains 79,999 83,896
Gross Unrealized Losses (7,460) (7,577)
Net Unrealized Gain/(Loss) 72,539 76,319
Non-Agency RMBS - Subordinated interest-only    
Investment Holdings [Line Items]    
Principal or Notional Value 274,357 284,931
Total Premium 14,527 15,226
Amortized Cost 14,527 15,226
Fair Value 12,553 11,254
Gross Unrealized Gains 83 62
Gross Unrealized Losses (2,057) (4,035)
Net Unrealized Gain/(Loss) (1,974) (3,973)
Agency MBS - Residential    
Investment Holdings [Line Items]    
Principal or Notional Value 2,833,811 5,045,418
Total Premium 142,459 255,837
Amortized Cost 2,976,270 5,301,255
Fair Value 3,029,836 5,267,848
Gross Unrealized Gains 53,860 18,593
Gross Unrealized Losses (294) (52,001)
Net Unrealized Gain/(Loss) 53,566 (33,408)
Agency MBS - Commercial    
Investment Holdings [Line Items]    
Principal or Notional Value 1,226,725 952,091
Total Premium 34,279 24,815
Total Discount (2,720) (3,170)
Amortized Cost 1,258,284 973,736
Fair Value 1,309,644 973,787
Gross Unrealized Gains 51,716 8,052
Gross Unrealized Losses (356) (8,001)
Net Unrealized Gain/(Loss) 51,360 51
Agency MBS - Interest-only    
Investment Holdings [Line Items]    
Principal or Notional Value 3,077,732 6,722,472
Total Premium 148,132 280,073
Amortized Cost 148,132 280,073
Fair Value 142,023 273,189
Gross Unrealized Gains 2,320 2,756
Gross Unrealized Losses (8,429) (9,640)
Net Unrealized Gain/(Loss) (6,109) (6,884)
Non-Agency RMBS - Senior    
Investment Holdings [Line Items]    
Principal or Notional Value 3,398,933 3,651,869
Total Premium 251 309
Total Discount (1,503,999) (1,553,317)
Amortized Cost 1,895,185 2,098,860
Fair Value 2,620,907 2,826,121
Gross Unrealized Gains 728,950 736,040
Gross Unrealized Losses (3,228) (8,779)
Net Unrealized Gain/(Loss) $ 725,722 $ 727,261