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Mortgage-Backed Securities - Summary (Detail) - Residential Mortgage-Backed Securities - USD ($)
$ in Thousands
Dec. 31, 2016
Dec. 31, 2015
Investment Holdings [Line Items]    
Principal or Notional Value $ 17,062,076 $ 22,845,276
Total Premium 662,686 868,410
Total Discount (1,627,480) (1,814,615)
Amortized Cost 6,825,525 9,465,640
Fair Value 7,497,817 10,190,665
Gross Unrealized Gains 841,967 867,512
Gross Unrealized Losses (169,675) (142,490)
Net Unrealized Gain/(Loss) 672,292 725,022
Non-Agency RMBS - Senior    
Investment Holdings [Line Items]    
Principal or Notional Value 3,190,947 3,651,869
Total Premium 231 309
Total Discount (1,412,058) (1,553,317)
Amortized Cost 1,779,120 2,098,860
Fair Value 2,511,003 2,826,121
Gross Unrealized Gains 732,133 736,040
Gross Unrealized Losses (250) (8,779)
Net Unrealized Gain/(Loss) 731,883 727,261
Non-Agency RMBS - Senior interest-only    
Investment Holdings [Line Items]    
Principal or Notional Value 5,648,339 5,426,029
Total Premium 292,396 268,515
Amortized Cost 292,396 268,515
Fair Value 253,539 234,171
Gross Unrealized Gains 18,674 18,113
Gross Unrealized Losses (57,531) (52,457)
Net Unrealized Gain/(Loss) (38,857) (34,344)
Non-Agency RMBS - Subordinated    
Investment Holdings [Line Items]    
Principal or Notional Value 673,259 762,466
Total Premium 16,352 23,635
Total Discount (212,734) (258,128)
Amortized Cost 476,877 527,975
Fair Value 553,498 604,295
Gross Unrealized Gains 77,857 83,896
Gross Unrealized Losses (1,236) (7,577)
Net Unrealized Gain/(Loss) 76,621 76,319
Non-Agency RMBS - Subordinated interest-only    
Investment Holdings [Line Items]    
Principal or Notional Value 266,927 284,931
Total Premium 13,878 15,226
Amortized Cost 13,878 15,226
Fair Value 12,024 11,254
Gross Unrealized Gains 0 62
Gross Unrealized Losses (1,854) (4,035)
Net Unrealized Gain/(Loss) (1,854) (3,973)
Agency MBS - Residential    
Investment Holdings [Line Items]    
Principal or Notional Value 2,594,570 5,045,418
Total Premium 149,872 255,837
Amortized Cost 2,744,442 5,301,255
Fair Value 2,705,978 5,267,848
Gross Unrealized Gains 11,235 18,593
Gross Unrealized Losses (49,699) (52,001)
Net Unrealized Gain/(Loss) (38,464) (33,408)
Agency MBS - Commercial    
Investment Holdings [Line Items]    
Principal or Notional Value 1,331,543 952,091
Total Premium 37,782 24,815
Total Discount (2,688) (3,170)
Amortized Cost 1,366,637 973,736
Fair Value 1,316,975 973,787
Gross Unrealized Gains 175 8,052
Gross Unrealized Losses (49,837) (8,001)
Net Unrealized Gain/(Loss) (49,662) 51
Agency MBS - Interest-only    
Investment Holdings [Line Items]    
Principal or Notional Value 3,356,491 6,722,472
Total Premium 152,175 280,073
Amortized Cost 152,175 280,073
Fair Value 144,800 273,189
Gross Unrealized Gains 1,893 2,756
Gross Unrealized Losses (9,268) (9,640)
Net Unrealized Gain/(Loss) $ (7,375) $ (6,884)