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Investments in Debt Securities Investments in Debt Securities (Details) $ in Thousands | 3 Months Ended |
--- | --- | ---
Mar. 31, 2020 USD ($) | Mar. 31, 2019 USD ($) | Dec. 31, 2019 USD ($)
Investments [Line Items] |  |  |  |  |
Par value of available for sale debt security | $ 2,974,762 |  |  | $ 4,455,808 |
Investments, Unamortized Premium (Discount) | 498,166 |  |  | 558,549 |
Debt Securities, Available-for-sale, Amortized Cost | 3,472,928 |  |  | 5,014,357 |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 254,529 |  |  | 175,157 |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | (7,751) |  |  | (1,351) |
Debt Securities, Available-for-sale | 3,719,706 |  |  | 5,188,163 |
Proceeds from Sale of Available-for-sale Securities | 1,991,034 |  | $ 0 |  |
Debt Securities, Available-for-sale, Realized Gain (Loss) | 84,783 |  | 0 |  |
Receivables from Brokers-Dealers and Clearing Organizations | 1,503,571 |  |  | 0 |
Securities sold, unsettled, at amortized cost | 1,431,025 |  |  |  |
Agency MBS |  |  |  |  |
Investments [Line Items] |  |  |  |  |
Notional balance for interest only securities | 13,274,419 |  |  | 13,404,824 |
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value | 322,607 |  |  | 215,792 |
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | $ 5,846 |  |  | $ 1,139 |
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | 56 |  |  | 27 |
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | $ 388 |  |  | $ 75,745 |
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | $ 79 |  |  | $ 35 |
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | 2 |  |  | 2 |
Interest-Only-Strip |  |  |  |  |
Investments [Line Items] |  |  |  |  |
Par value of available for sale debt security | $ 0 | [1] |  | $ 0 | [2]
Investments, Unamortized Premium (Discount) | 462,675 |  |  | 488,145 |
Debt Securities, Available-for-sale, Amortized Cost | 462,675 |  |  | 488,145 |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 2,616 |  |  | 11,760 |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | (6,802) |  |  | (863) |
Debt Securities, Available-for-sale | 458,489 |  |  | 499,042 |
Non-Agency MBS |  |  |  |  |
Investments [Line Items] |  |  |  |  |
Par value of available for sale debt security | 1,833 |  |  | 1,938 |
Investments, Unamortized Premium (Discount) | (686) |  |  | (780) |
Notional balance for interest only securities | 9,680,637 |  |  | 9,799,629 |
Debt Securities, Available-for-sale, Amortized Cost | 1,147 |  |  | 1,158 |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 472 |  |  | 552 |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | (57) |  |  | (20) |
Debt Securities, Available-for-sale | 1,562 |  |  | 1,690 |
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value | 115,425 |  |  | 13,607 |
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | $ 1,804 |  |  | $ 146 |
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | 51 |  |  | 7 |
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | $ 111 |  |  | $ 1,099 |
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | $ 22 |  |  | $ 31 |
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | 4 |  |  | 5 |
Agency | Commercial Mortgage Backed Securities |  |  |  |  |
Investments [Line Items] |  |  |  |  |
Par value of available for sale debt security | $ 2,104,738 |  |  | $ 1,890,186 |
Investments, Unamortized Premium (Discount) | 16,937 |  |  | 15,414 |
Debt Securities, Available-for-sale, Amortized Cost | 2,121,675 |  |  | 1,905,600 |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 207,436 |  |  | 93,763 |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | (892) |  |  | (6) |
Debt Securities, Available-for-sale | 2,328,219 |  |  | 1,999,357 |
Proceeds from Sale of Available-for-sale Securities | 173,684 |  | 0 |  |
Debt Securities, Available-for-sale, Realized Loss | 20,689 |  | 0 |  |
Agency | Residential Mortgage Backed Securities |  |  |  |  |
Investments [Line Items] |  |  |  |  |
Par value of available for sale debt security | 868,191 |  |  | 2,563,684 |
Investments, Unamortized Premium (Discount) | 19,240 |  |  | 55,770 |
Debt Securities, Available-for-sale, Amortized Cost | 887,431 |  |  | 2,619,454 |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 44,005 |  |  | 69,082 |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 0 |  |  | (462) |
Debt Securities, Available-for-sale | 931,436 |  |  | $ 2,688,074 |
Proceeds from Sale of Available-for-sale Securities | 1,817,350 |  | 0 |  |
Debt Securities, Available-for-sale, Realized Gain | $ 64,094 |  |  |  |
Debt Securities, Available-for-sale, Realized Loss |  |  | $ 0 |  |
[1] | The notional balance for the Agency CMBS IO and non-Agency CMBS IO was $13,404,824 and $9,799,629, respectively, as of December 31, 2019.
[2] | The notional balance for Agency CMBS IO and non-Agency CMBS IO was $13,274,419 and $9,680,637 respectively, as of March 31, 2020.

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