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Mortgage-Backed Securities - Collateral Characteristics (Detail) - Residential Mortgage-Backed Securities
$ in Thousands
Dec. 31, 2017
USD ($)
$ / security
Dec. 31, 2016
USD ($)
$ / security
Investment Holdings [Line Items]    
Principal or Notional Value at Period-End | $ $ 15,442,782 $ 17,062,076
Non-Agency RMBS - Senior    
Investment Holdings [Line Items]    
Principal or Notional Value at Period-End | $ $ 2,733,926 $ 3,190,947
Weighted Average Amortized Cost Basis 54.04 55.76
Weighted Average Fair Value 81.62 78.69
Weighted Average Coupon 4.60% 4.30%
Weighted Average Yield at Period-End 16.70% 15.50%
Non-Agency RMBS - Senior interest-only    
Investment Holdings [Line Items]    
Principal or Notional Value at Period-End | $ $ 4,862,461 $ 5,648,339
Weighted Average Amortized Cost Basis 5.41 5.18
Weighted Average Fair Value 4.34 4.49
Weighted Average Coupon 1.30% 1.50%
Weighted Average Yield at Period-End 8.00% 11.70%
Non-Agency RMBS - Subordinated    
Investment Holdings [Line Items]    
Principal or Notional Value at Period-End | $ $ 501,455 $ 673,259
Weighted Average Amortized Cost Basis 66.77 70.83
Weighted Average Fair Value 80.01 82.21
Weighted Average Coupon 4.10% 3.80%
Weighted Average Yield at Period-End 9.60% 9.20%
Non-Agency RMBS - Subordinated interest-only    
Investment Holdings [Line Items]    
Principal or Notional Value at Period-End | $ $ 201,378 $ 266,927
Weighted Average Amortized Cost Basis 3.66 5.20
Weighted Average Fair Value 3.89 4.50
Weighted Average Coupon 0.80% 1.10%
Weighted Average Yield at Period-End 11.80% 13.50%
Agency MBS - Residential    
Investment Holdings [Line Items]    
Principal or Notional Value at Period-End | $ $ 2,227,128 $ 2,594,570
Weighted Average Amortized Cost Basis 105.53 105.78
Weighted Average Fair Value 104.27 104.29
Weighted Average Coupon 3.80% 3.90%
Weighted Average Yield at Period-End 2.90% 3.00%
Agency MBS - Commercial    
Investment Holdings [Line Items]    
Principal or Notional Value at Period-End | $ $ 1,894,594 $ 1,331,543
Weighted Average Amortized Cost Basis 102.26 102.64
Weighted Average Fair Value 102.31 98.91
Weighted Average Coupon 3.60% 3.60%
Weighted Average Yield at Period-End 3.20% 2.90%
Agency MBS - Interest-only    
Investment Holdings [Line Items]    
Principal or Notional Value at Period-End | $ $ 3,021,840 $ 3,356,491
Weighted Average Amortized Cost Basis 3.68 4.53
Weighted Average Fair Value 3.45 4.31
Weighted Average Coupon 0.70% 0.80%
Weighted Average Yield at Period-End 3.40% 3.50%