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Investments in Debt Securities Investments in Debt Securities (Details) | 12 Months Ended
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Dec. 31, 2020 USD ($) | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($)
Investments [Line Items] |  |  |  |  |
Par value of available for sale debt security | $ 2,076,346,000 |  | $ 4,455,808,000 |  |
Investments, Unamortized Premium (Discount) | 439,649,000 |  | 558,549,000 |  |
Debt Securities, Available-for-sale, Amortized Cost | 2,515,995,000 |  | 5,014,357,000 |  |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 81,293,000 |  | 175,157,000 |  |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | (1,033,000) |  | (1,351,000) |  |
Debt Securities, Available-for-sale | 2,596,255,000 |  | 5,188,163,000 |  |
Proceeds from Sale of Available-for-sale Securities | 4,642,305,000 |  | 1,033,066,000 |  | $ 733,064,000
Debt Securities, Available-for-sale, Realized Gain (Loss) | 308,084,000 |  | (5,755,000) |  | (23,373,000)
Debt Securities, Available-for-sale, Allowance for Credit Loss | 0 |  |  |  |
Agency MBS |  |  |  |  |
Investments [Line Items] |  |  |  |  |
Notional balance for interest only securities | 11,277,908,000 |  | 13,404,824,000 |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value | 19,266,000 |  | 215,792,000 |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | $ 399,000 |  | $ 1,139,000 |  |
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | 19 |  | 27 |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | $ 749,000 |  | $ 75,745,000 |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | $ 133,000 |  | $ 35,000 |  |
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) | 378,940,000 |  | 488,145,000 |  |
Debt Securities, Available-for-sale, Amortized Cost | 378,940,000 |  | 488,145,000 |  |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 12,081,000 |  | 11,760,000 |  |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | (982,000) |  | (863,000) |  |
Debt Securities, Available-for-sale | 390,039,000 |  | 499,042,000 |  |
Non-Agency MBS |  |  |  |  |
Investments [Line Items] |  |  |  |  |
Par value of available for sale debt security | 1,499,000 |  | 1,938,000 |  |
Investments, Unamortized Premium (Discount) | (440,000) |  | (780,000) |  |
Notional balance for interest only securities | 9,319,520,000 |  | 9,799,629,000 |  |
Debt Securities, Available-for-sale, Amortized Cost | 1,059,000 |  | 1,158,000 |  |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 267,000 |  | 552,000 |  |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | (51,000) |  | (20,000) |  |
Debt Securities, Available-for-sale | 1,275,000 |  | 1,690,000 |  |
Proceeds from Sale of Available-for-sale Securities |  |  |  |  | 8,695,000
Debt Securities, Available-for-sale, Realized Loss |  |  |  |  | 51,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value | 33,417,000 |  | 13,607,000 |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss | $ 408,000 |  | $ 146,000 |  |
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | 23 |  | 7 |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value | $ 2,156,000 |  | $ 1,099,000 |  |
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss | $ 93,000 |  | $ 31,000 |  |
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | 5 |  | 5 |  |
U.S. Treasuries |  |  |  |  |
Investments [Line Items] |  |  |  |  |
Proceeds from Sale of Available-for-sale Securities |  |  |  |  | 248,803,000
Debt Securities, Available-for-sale, Realized Loss |  |  |  |  | (6,567,000)
Agency | Commercial Mortgage Backed Securities |  |  |  |  |
Investments [Line Items] |  |  |  |  |
Par value of available for sale debt security | $ 235,801,000 |  | $ 1,890,186,000 |  |
Investments, Unamortized Premium (Discount) | 3,152,000 |  | 15,414,000 |  |
Debt Securities, Available-for-sale, Amortized Cost | 238,953,000 |  | 1,905,600,000 |  |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 19,597,000 |  | 93,763,000 |  |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 0 |  | (6,000) |  |
Debt Securities, Available-for-sale | 258,550,000 |  | 1,999,357,000 |  |
Proceeds from Sale of Available-for-sale Securities | 2,247,273,000 |  | 213,199,000 |  | 242,029,000
Debt Securities, Available-for-sale, Realized Loss | 225,395,000 |  | (6,493,000) |  | (9,218,000)
Agency | Residential Mortgage Backed Securities |  |  |  |  |
Investments [Line Items] |  |  |  |  |
Par value of available for sale debt security | 1,839,046,000 |  | 2,563,684,000 |  |
Investments, Unamortized Premium (Discount) | 57,997,000 |  | 55,770,000 |  |
Debt Securities, Available-for-sale, Amortized Cost | 1,897,043,000 |  | 2,619,454,000 |  |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 49,348,000 |  | 69,082,000 |  |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 0 |  | (462,000) |  |
Debt Securities, Available-for-sale | 1,946,391,000 |  | 2,688,074,000 |  |
Proceeds from Sale of Available-for-sale Securities | 2,395,032,000 |  | 796,699,000 |  | 217,837,000
Debt Securities, Available-for-sale, Realized Gain | $ 82,689,000 |  |  |  |
Debt Securities, Available-for-sale, Realized Loss |  |  | 506,000 |  | (7,785,000)
Agency | Interest-Only-Strip |  |  |  |  |
Investments [Line Items] |  |  |  |  |
Proceeds from Sale of Available-for-sale Securities |  |  | 23,168,000 |  | 15,700,000
Debt Securities, Available-for-sale, Realized Loss |  |  | $ 232,000 |  | $ 146,000
[1] | The notional balance for Agency CMBS IO and non-Agency CMBS IO was $11,277,908 and $9,319,520 respectively, as of December 31, 2020.
[2] | 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.

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- Definition The remaining amount of investment premium (discount), net, originally recognized at the purchase date of the instruments that has yet to be amortized (accreted) into earnings. + References No definition available. + Details
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- Definition Notional balance for interest only securities owned by the Company. Interest only securities represent the right to receive excess interest payments, and not principal cash flows, based on the underlying unpaid principal balance (ie, notional) of the underlying pool of mortgage loans. + References No definition available. + Details
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