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Financing Agreements (Financing Agreements) (Details) - USD ($) $ in Thousands | Dec. 31, 2020 | Jun. 26, 2020 | Dec. 31, 2019
--- | --- | --- | ---
Financing Agreements [Line Items] |  |  |
Unpaid Principal Balance | $ 4,339,479 |  | $ 10,044,844
Fair value / Carrying Value | 4,336,976 |  | 10,031,606
Financing agreements with non-mark-to-market collateral provisions |  |  |
Financing Agreements [Line Items] |  |  |
Unpaid Principal Balance | 1,156,899 |  |
Amortized Cost Balance | 1,156,899 |  |
Fair value / Carrying Value | 1,159,213 |  |
Financing agreements with mark-to-market collateral provisions |  |  |
Financing Agreements [Line Items] |  |  |
Unpaid Principal Balance | 1,338,077 |  |
Amortized Cost Balance | 1,338,077 |  |
Fair value / Carrying Value | 1,338,077 | $ 1,650,000 |
Securitized debt |  |  |
Financing Agreements [Line Items] |  |  |
Unpaid Principal Balance | 866,203 |  | 573,900
Amortized Cost Balance | 857,553 |  |
Fair value / Carrying Value | 869,482 |  | 570,952
Total Financing agreements, at fair value |  |  |
Financing Agreements [Line Items] |  |  |
Unpaid Principal Balance | 3,361,179 |  |
Amortized Cost Balance | 3,352,529 |  |
Fair value / Carrying Value | 3,366,772 |  |
Repurchase agreements |  |  |
Financing Agreements [Line Items] |  |  |
Unpaid Principal Balance |  |  | 9,140,944
Fair value / Carrying Value |  |  | 9,139,821
Securitized debt |  |  |
Financing Agreements [Line Items] |  |  |
Unpaid Principal Balance | 648,300 |  |
Fair value / Carrying Value | 645,027 |  |
Convertible senior notes |  |  |
Financing Agreements [Line Items] |  |  |
Unpaid Principal Balance | 230,000 |  | 230,000
Fair value / Carrying Value | 225,177 |  | 223,971
Senior Notes |  |  |
Financing Agreements [Line Items] |  |  |
Unpaid Principal Balance | 100,000 |  | 100,000
Fair value / Carrying Value | 100,000 |  | $ 96,862
Total Financing agreements at carrying value |  |  |
Financing Agreements [Line Items] |  |  |
Unpaid Principal Balance | 978,300 |  |
Fair value / Carrying Value | $ 970,204 |  |

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