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Mortgage-Backed Securities - Investment Type (Details) - USD ($) $ in Thousands | Mar. 31, 2024 | Dec. 31, 2023
--- | --- | ---
Debt Securities, Trading and Available-for-sale [Abstract] |  |  |
Par Value |  | $ 6,046,940 | $ 6,143,945
Amortized Cost |  | 6,172,124 | 6,283,185
Fair Value |  | 5,840,559 | 6,038,948
Amortized cost |  | 1,099,142 | 1,131,769
Gross Unrealized Gain |  | 1,505 | 1,324
Gross Unrealized Loss |  | (177,275) | (159,826)
Fair Value |  | 923,372 | 973,267
Amortized Cost |  | 5,072,982 | 5,151,416
Gross Unrealized Gain |  | 22,675 | 48,461
Gross Unrealized Loss |  | (178,470) | (134,196)
Fair Value |  | 4,917,187 | 5,065,681
Residential Mortgage Backed Securities [Member] | Agency MBS |  |  |
Debt Securities, Trading and Available-for-sale [Abstract] |  |  |
Par Value |  | 5,928,956 | 6,022,502
Amortized Cost |  | 5,900,650 | 5,993,922
Fair Value |  | 5,583,208 | 5,763,532
Amortized cost |  | 882,334 | 898,420
Gross Unrealized Gain |  | 0 | 0
Gross Unrealized Loss |  | (165,983) | (148,606)
Fair Value |  | 716,351 | 749,814
Amortized Cost |  | 5,018,316 | 5,095,502
Gross Unrealized Gain |  | 22,675 | 48,459
Gross Unrealized Loss |  | (174,134) | (130,243)
Fair Value |  | 4,866,857 | 5,013,718
Commercial Mortgage Backed Securities [Member] | Agency MBS |  |  |
Debt Securities, Trading and Available-for-sale [Abstract] |  |  |
Par Value |  | 117,984 | 121,293
Amortized Cost |  | 118,473 | 121,799
Fair Value |  | 111,762 | 115,595
Amortized cost |  | 103,209 | 106,527
Gross Unrealized Gain |  | 0 | 28
Gross Unrealized Loss |  | (5,448) | (5,159)
Fair Value |  | 97,761 | 101,396
Amortized Cost |  | 15,264 | 15,272
Gross Unrealized Gain |  | 0 | 0
Gross Unrealized Loss |  | (1,263) | (1,073)
Fair Value |  | 14,001 | 14,199
Interest-Only-Strip [Member] |  |  |
Debt Securities, Trading and Available-for-sale [Abstract] |  |  |
Amortized Cost | [1] | 153,001 | 167,314
Fair Value | [1] | 145,589 | 159,718
Amortized cost |  | 113,599 | 126,672
Gross Unrealized Gain |  | 1,505 | 1,296
Gross Unrealized Loss |  | (5,844) | (6,014)
Fair Value |  | 109,260 | 121,954
Amortized Cost |  | 39,402 | 40,642
Gross Unrealized Gain |  | 0 | 2
Gross Unrealized Loss |  | (3,073) | (2,880)
Fair Value |  | 36,329 | 37,764
Interest-Only-Strip, Agency CMBS [Member] |  |  |
Debt Securities, Trading and Available-for-sale [Abstract] |  |  |
Notional balance |  | 7,594,510 | 7,723,379
Interest-Only-Strip, Non-Agency CMBS [Member] |  |  |
Debt Securities, Trading and Available-for-sale [Abstract] |  |  |
Notional balance |  | 3,529,181 | 3,860,007
Non-Agency MBS |  |  |
Debt Securities, Trading and Available-for-sale [Abstract] |  |  |
Par Value |  | 0 | 150
Fair Value |  | 0 | 103
Amortized cost |  | 0 | 150
Gross Unrealized Gain |  | 0 | 0
Gross Unrealized Loss |  | 0 | (47)
Fair Value |  | $ 0 | $ 103
[1] | (1) The notional balance for Agency CMBS IO and non-Agency CMBS IO was $7,594,510 and $3,529,181, respectively, as of March 31, 2024, and $7,723,379 and $3,860,007, respectively, as of December 31, 2023.

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