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Mortgage-Backed Securities - Summary (Detail) - Residential Mortgage-Backed Securities - USD ($)
$ in Thousands
Jun. 30, 2017
Dec. 31, 2016
Investment Holdings [Line Items]    
Principal or Notional Value $ 15,970,359 $ 17,062,076
Total Premium 608,093 662,686
Total Discount (1,526,241) (1,627,480)
Amortized Cost 6,390,109 6,825,525
Fair Value 7,106,497 7,497,817
Gross Unrealized Gains 857,749 841,967
Gross Unrealized Losses (141,361) (169,675)
Net Unrealized Gain/(Loss) 716,388 672,292
Non-Agency RMBS - Senior    
Investment Holdings [Line Items]    
Principal or Notional Value 2,919,136 3,190,947
Total Premium 211 231
Total Discount (1,317,988) (1,412,058)
Amortized Cost 1,601,359 1,779,120
Fair Value 2,353,109 2,511,003
Gross Unrealized Gains 751,959 732,133
Gross Unrealized Losses (209) (250)
Net Unrealized Gain/(Loss) 751,750 731,883
Non-Agency RMBS - Senior interest-only    
Investment Holdings [Line Items]    
Principal or Notional Value 5,140,383 5,648,339
Total Premium 276,574 292,396
Total Discount 0 0
Amortized Cost 276,574 292,396
Fair Value 219,488 253,539
Gross Unrealized Gains 13,403 18,674
Gross Unrealized Losses (70,489) (57,531)
Net Unrealized Gain/(Loss) (57,086) (38,857)
Non-Agency RMBS - Subordinated    
Investment Holdings [Line Items]    
Principal or Notional Value 646,779 673,259
Total Premium 12,737 16,352
Total Discount (205,659) (212,734)
Amortized Cost 453,857 476,877
Fair Value 528,372 553,498
Gross Unrealized Gains 75,838 77,857
Gross Unrealized Losses (1,323) (1,236)
Net Unrealized Gain/(Loss) 74,515 76,621
Non-Agency RMBS - Subordinated interest-only    
Investment Holdings [Line Items]    
Principal or Notional Value 258,908 266,927
Total Premium 13,286 13,878
Total Discount 0 0
Amortized Cost 13,286 13,878
Fair Value 10,859 12,024
Gross Unrealized Gains 223 0
Gross Unrealized Losses (2,650) (1,854)
Net Unrealized Gain/(Loss) (2,427) (1,854)
Agency MBS - Residential    
Investment Holdings [Line Items]    
Principal or Notional Value 2,376,069 2,594,570
Total Premium 136,783 149,872
Total Discount 0 0
Amortized Cost 2,512,852 2,744,442
Fair Value 2,485,612 2,705,978
Gross Unrealized Gains 10,324 11,235
Gross Unrealized Losses (37,564) (49,699)
Net Unrealized Gain/(Loss) (27,240) (38,464)
Agency MBS - Commercial    
Investment Holdings [Line Items]    
Principal or Notional Value 1,366,273 1,331,543
Total Premium 38,765 37,782
Total Discount (2,594) (2,688)
Amortized Cost 1,402,444 1,366,637
Fair Value 1,385,084 1,316,975
Gross Unrealized Gains 4,514 175
Gross Unrealized Losses (21,874) (49,837)
Net Unrealized Gain/(Loss) (17,360) (49,662)
Agency MBS - Interest-only    
Investment Holdings [Line Items]    
Principal or Notional Value 3,262,811 3,356,491
Total Premium 129,737 152,175
Total Discount 0 0
Amortized Cost 129,737 152,175
Fair Value 123,973 144,800
Gross Unrealized Gains 1,488 1,893
Gross Unrealized Losses (7,252) (9,268)
Net Unrealized Gain/(Loss) $ (5,764) $ (7,375)