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Mortgage-Backed Securities - Summary (Detail) - USD ($)
$ in Thousands
Dec. 31, 2019
Dec. 31, 2018
Investment Holdings [Line Items]    
Principal or Notional Value $ 22,419,481 $ 23,577,333
Total Premium 687,503 725,412
Total Discount (1,291,790) (1,296,506)
Amortized Cost 10,999,354 14,088,920
Fair Value 11,955,418 14,675,080
Gross Unrealized Gains 1,042,048 816,456
Gross Unrealized Losses (85,984) (230,296)
Net Unrealized Gain/(Loss) 956,064 586,160
Residential Mortgage-Backed Securities | Non-Agency RMBS - Senior    
Investment Holdings [Line Items]    
Principal or Notional Value 2,024,564 2,386,049
Total Premium 2,038 537
Total Discount (953,916) (1,112,368)
Amortized Cost 1,072,686 1,274,218
Fair Value 1,700,911 1,943,124
Gross Unrealized Gains 628,518 669,356
Gross Unrealized Losses (293) (450)
Net Unrealized Gain/(Loss) 628,225 668,906
Residential Mortgage-Backed Securities | Non-Agency RMBS - Subordinated    
Investment Holdings [Line Items]    
Principal or Notional Value 876,592 394,037
Total Premium 9,915 8,642
Total Discount (332,913) (179,669)
Amortized Cost 553,594 223,010
Fair Value 624,598 276,467
Gross Unrealized Gains 76,272 53,702
Gross Unrealized Losses (5,268) (245)
Net Unrealized Gain/(Loss) 71,004 53,457
Residential Mortgage-Backed Securities | Non-Agency RMBS - Senior interest-only    
Investment Holdings [Line Items]    
Principal or Notional Value 7,458,653 5,888,747
Total Premium 301,170 296,874
Total Discount 0 0
Amortized Cost 301,170 296,874
Fair Value 288,899 266,539
Gross Unrealized Gains 51,481 33,123
Gross Unrealized Losses (63,752) (63,458)
Net Unrealized Gain/(Loss) (12,271) (30,335)
Residential Mortgage-Backed Securities | Agency MBS - Residential    
Investment Holdings [Line Items]    
Principal or Notional Value 6,080,547 8,984,249
Total Premium 131,023 221,606
Total Discount 0 0
Amortized Cost 6,211,570 9,205,855
Fair Value 6,362,626 9,174,382
Gross Unrealized Gains 152,271 51,986
Gross Unrealized Losses (1,215) (83,459)
Net Unrealized Gain/(Loss) 151,056 (31,473)
Residential Mortgage-Backed Securities | Agency RMBS - Interest-only    
Investment Holdings [Line Items]    
Principal or Notional Value 1,539,941 578,284
Total Premium 139,536 64,185
Total Discount 0 0
Amortized Cost 139,536 64,185
Fair Value 127,667 65,675
Gross Unrealized Gains 220 1,490
Gross Unrealized Losses (12,089) 0
Net Unrealized Gain/(Loss) (11,869) 1,490
Commercial Mortgage Backed Securities | Agency MBS - Commercial    
Investment Holdings [Line Items]    
Principal or Notional Value 2,621,938 2,895,679
Total Premium 52,681 61,727
Total Discount (4,961) (4,469)
Amortized Cost 2,669,658 2,952,937
Fair Value 2,801,692 2,881,222
Gross Unrealized Gains 132,700 6,303
Gross Unrealized Losses (666) (78,018)
Net Unrealized Gain/(Loss) 132,034 (71,715)
Commercial Mortgage Backed Securities | Agency CMBS - Interest-only    
Investment Holdings [Line Items]    
Principal or Notional Value 1,817,246 2,450,288
Total Premium 51,140 71,841
Total Discount 0 0
Amortized Cost 51,140 71,841
Fair Value 49,025 67,671
Gross Unrealized Gains 586 496
Gross Unrealized Losses (2,701) (4,666)
Net Unrealized Gain/(Loss) $ (2,115) $ (4,170)