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Mortgage-Backed Securities - Investment Type (Details) - USD ($) $ in Thousands | Sep. 30, 2023 | Dec. 31, 2022
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Debt Securities, Trading and Available-for-sale [Abstract] |  |  |
Par Value |  | $ 5,975,467 | $ 3,236,285
Amortized Cost |  | 6,151,247 | 3,522,256
Fair Value |  | 5,583,757 | 3,112,705
Amortized cost |  | 1,164,265 | 1,288,269
Gross Unrealized Gain |  | 1,275 | 507
Gross Unrealized Loss |  | (219,045) | (181,853)
Fair Value |  | 946,495 | 1,106,923
Amortized Cost |  | 4,986,982 | 2,233,987
Gross Unrealized Gain |  | 0 | 0
Gross Unrealized Loss |  | (349,720) | (228,205)
Fair Value |  | 4,637,262 | 2,005,782
Residential Mortgage Backed Securities [Member] |  |  |
Debt Securities, Trading and Available-for-sale [Abstract] |  |  |
Par Value |  | 5,853,691 | 3,104,498
Amortized Cost |  | 5,844,730 | 3,150,873
Fair Value |  | 5,298,271 | 2,762,878
Amortized cost |  | 914,900 | 977,624
Gross Unrealized Gain |  | 0 | 0
Gross Unrealized Loss |  | (202,677) | (164,949)
Fair Value |  | 712,223 | 812,675
Amortized Cost |  | 4,929,830 | 2,173,249
Gross Unrealized Gain |  | 0 | 0
Gross Unrealized Loss |  | (343,782) | (223,046)
Fair Value |  | 4,586,048 | 1,950,203
Commercial Mortgage Backed Securities [Member] |  |  |
Debt Securities, Trading and Available-for-sale [Abstract] |  |  |
Par Value |  | 121,617 | 131,578
Amortized Cost |  | 122,171 | 132,333
Fair Value |  | 112,396 | 124,690
Amortized cost |  | 106,892 | 117,031
Gross Unrealized Gain |  | 0 | 0
Gross Unrealized Loss |  | (7,791) | (6,474)
Fair Value |  | 99,101 | 110,557
Amortized Cost |  | 15,279 | 15,302
Gross Unrealized Gain |  | 0 | 0
Gross Unrealized Loss |  | (1,984) | (1,169)
Fair Value |  | 13,295 | 14,133
Interest-Only-Strip [Member] |  |  |
Debt Securities, Trading and Available-for-sale [Abstract] |  |  |
Amortized Cost | [1] | 184,187 | 238,841
Fair Value | [1] | 172,987 | 224,985
Amortized cost |  | 142,314 | 193,405
Gross Unrealized Gain |  | 1,275 | 507
Gross Unrealized Loss |  | (8,521) | (10,373)
Fair Value |  | 135,068 | 183,539
Amortized Cost |  | 41,873 | 45,436
Gross Unrealized Gain |  | 0 | 0
Gross Unrealized Loss |  | (3,954) | (3,990)
Fair Value |  | 37,919 | 41,446
Interest-Only-Strip, Agency CMBS [Member] |  |  |
Debt Securities, Trading and Available-for-sale [Abstract] |  |  |
Notional balance |  | 7,814,762 | 9,711,981
Interest-Only-Strip, Non-Agency CMBS [Member] |  |  |
Debt Securities, Trading and Available-for-sale [Abstract] |  |  |
Notional balance |  | 4,159,373 | 6,280,761
Non-Agency MBS |  |  |
Debt Securities, Trading and Available-for-sale [Abstract] |  |  |
Par Value |  | 159 | 209
Fair Value |  | 103 | 152
Amortized cost |  | 159 | 209
Gross Unrealized Gain |  | 0 | 0
Gross Unrealized Loss |  | (56) | (57)
Fair Value |  | $ 103 | $ 152
[1] | (1) The notional balance for Agency CMBS IO and non-Agency CMBS IO was $7,814,762 and $4,159,373, respectively, as of September 30, 2023, and $9,711,981 and $6,280,761, respectively, as of December 31, 2022.

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