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Mortgage-Backed Securities - Summary (Detail) - Residential Mortgage-Backed Securities - USD ($)
$ in Thousands
Mar. 31, 2017
Dec. 31, 2016
Investment Holdings [Line Items]    
Principal or Notional Value $ 16,542,679 $ 17,062,076
Total Premium 637,784 662,686
Total Discount (1,577,123) (1,627,480)
Amortized Cost 6,657,644 6,825,525
Fair Value 7,330,242 7,497,817
Gross Unrealized Gains 843,660 841,967
Gross Unrealized Losses (171,062) (169,675)
Net Unrealized Gain/(Loss) 672,598 672,292
Non-Agency RMBS - Senior    
Investment Holdings [Line Items]    
Principal or Notional Value 3,060,690 3,190,947
Total Premium 214 231
Total Discount (1,361,946) (1,412,058)
Amortized Cost 1,698,958 1,779,120
Fair Value 2,438,623 2,511,003
Gross Unrealized Gains 739,946 732,133
Gross Unrealized Losses (281) (250)
Net Unrealized Gain/(Loss) 739,665 731,883
Non-Agency RMBS - Senior interest-only    
Investment Holdings [Line Items]    
Principal or Notional Value 5,434,402 5,648,339
Total Premium 287,220 292,396
Amortized Cost 287,220 292,396
Fair Value 239,511 253,539
Gross Unrealized Gains 16,095 18,674
Gross Unrealized Losses (63,804) (57,531)
Net Unrealized Gain/(Loss) (47,709) (38,857)
Non-Agency RMBS - Subordinated    
Investment Holdings [Line Items]    
Principal or Notional Value 662,469 673,259
Total Premium 15,360 16,352
Total Discount (212,416) (212,734)
Amortized Cost 465,413 476,877
Fair Value 538,459 553,498
Gross Unrealized Gains 74,739 77,857
Gross Unrealized Losses (1,693) (1,236)
Net Unrealized Gain/(Loss) 73,046 76,621
Non-Agency RMBS - Subordinated interest-only    
Investment Holdings [Line Items]    
Principal or Notional Value 263,126 266,927
Total Premium 13,627 13,878
Amortized Cost 13,627 13,878
Fair Value 11,798 12,024
Gross Unrealized Gains 103 0
Gross Unrealized Losses (1,932) (1,854)
Net Unrealized Gain/(Loss) (1,829) (1,854)
Agency MBS - Residential    
Investment Holdings [Line Items]    
Principal or Notional Value 2,480,534 2,594,570
Total Premium 144,287 149,872
Amortized Cost 2,624,821 2,744,442
Fair Value 2,587,928 2,705,978
Gross Unrealized Gains 10,566 11,235
Gross Unrealized Losses (47,459) (49,699)
Net Unrealized Gain/(Loss) (36,893) (38,464)
Agency MBS - Commercial    
Investment Holdings [Line Items]    
Principal or Notional Value 1,393,290 1,331,543
Total Premium 37,763 37,782
Total Discount (2,761) (2,688)
Amortized Cost 1,428,292 1,366,637
Fair Value 1,382,734 1,316,975
Gross Unrealized Gains 1,334 175
Gross Unrealized Losses (46,892) (49,837)
Net Unrealized Gain/(Loss) (45,558) (49,662)
Agency MBS - Interest-only    
Investment Holdings [Line Items]    
Principal or Notional Value 3,248,168 3,356,491
Total Premium 139,313 152,175
Amortized Cost 139,313 152,175
Fair Value 131,189 144,800
Gross Unrealized Gains 877 1,893
Gross Unrealized Losses (9,001) (9,268)
Net Unrealized Gain/(Loss) $ (8,124) $ (7,375)