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Derivatives (Details) - USD ($) $ in Thousands |  | 3 Months Ended | 12 Months Ended
--- | --- | --- | ---
Dec. 31, 2019 | Mar. 31, 2021 | Mar. 31, 2020 | Dec. 31, 2020
Derivative [Line Items] |  |  |  |
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net |  | $ 107,801 | $ (195,567) |
Derivative Liability, Fair Value, Gross Liability |  | 19,866 |  | $ 1,634
Derivative assets, fair value |  | 109,746 |  | 11,342
Derivative liabilities, fair value |  | (19,866) |  | (1,634)
Interest rate swaps |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative Instruments Not Designated as Hedging Instruments, Loss |  | 0 | (182,181) |
Interest rate swaption |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative Instruments Not Designated as Hedging Instruments, Gain |  | 57,763 |  |
Derivative Instruments Not Designated as Hedging Instruments, Loss |  |  | (573) |
Derivative instrument, cost basis |  | $ 6,688 |  | 13,000
Derivative Liability, Fair Value, Gross Liability |  |  |  | $ 107
Derivative, Average Fixed Interest Rate |  | 1.16% |  | 1.07%
Derivative Asset, Notional Amount |  | $ 500,000 |  | $ 1,250,000
Derivative assets, fair value |  | 27,843 |  | 1,253
U.S. Treasury futures |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative Instruments Not Designated as Hedging Instruments, Gain |  | 95,647 |  |
Derivative Instruments Not Designated as Hedging Instruments, Loss |  |  | (8,447) |
Options on U.S. Treasury futures |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative Instruments Not Designated as Hedging Instruments, Gain |  | 12,617 |  |
Derivative Instruments Not Designated as Hedging Instruments, Loss |  |  | (10,727) |
Long position | TBA securities |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative Instruments Not Designated as Hedging Instruments, Gain |  |  | 18,432 |
Derivative Instruments Not Designated as Hedging Instruments, Loss |  | (58,226) |  |
Derivative Liability, Fair Value, Gross Liability |  | (19,866) |  |
Short position | U.S. Treasury futures |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative Liability, Fair Value, Gross Liability |  |  |  | 1,527
Short position | TBA securities |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative Instruments Not Designated as Hedging Instruments, Loss |  | 0 | $ (12,071) |
Not Designated as Hedging Instrument, Economic Hedge | Interest rate swaption |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative Asset, Notional Amount |  | 500,000 |  | 1,250,000
Derivative assets, fair value |  | 27,843 |  | 1,360
Derivative liabilities, fair value |  |  |  | (107)
Not Designated as Hedging Instrument, Economic Hedge | Interest rate swaption | 6 months or less |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative instrument, cost basis |  | $ 2,963 |  | $ 6,312
Derivative, Average Fixed Interest Rate |  | 1.12% |  | 1.02%
Derivative Asset, Notional Amount |  | $ 250,000 |  | $ 750,000
Derivative assets, fair value |  | 14,208 |  | 1,161
Not Designated as Hedging Instrument, Economic Hedge | Interest rate swaption | 6-9 months |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative instrument, cost basis |  | $ 3,725 |  | $ 6,688
Derivative, Average Fixed Interest Rate |  | 1.19% |  | 1.16%
Derivative Asset, Notional Amount |  | $ 250,000 |  | $ 500,000
Derivative assets, fair value |  | 13,635 |  | 92
Not Designated as Hedging Instrument, Economic Hedge | Options on U.S. Treasury futures |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative Asset, Notional Amount |  | 250,000 |  | 500,000
Derivative assets, fair value |  | $ 9,180 |  | 1,094
Derivative, Average Remaining Maturity | 1 month | 2 months |  |
Not Designated as Hedging Instrument, Economic Hedge | Short position | U.S. Treasury futures |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative Liability, Notional Amount |  | $ 2,980,000 |  | 825,000
Derivative assets, fair value |  | 72,723 |  |
Derivative liabilities, fair value |  | $ 0 |  | $ (1,527)
Derivative, Average Remaining Maturity |  | 3 months |  | 2 months
Not Designated as Hedging Instrument, Trading | Long position | TBA securities |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative Asset, Notional Amount |  | $ 2,800,000 |  | $ 1,515,000
Derivative assets, fair value |  |  |  | $ 8,888
Derivative liabilities, fair value |  | $ (19,866) |  |

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