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Financing Agreements (Financing Agreements) (Details) - USD ($) $ in Thousands | Mar. 31, 2021 | Dec. 31, 2020 | Jun. 30, 2020
--- | --- | --- | ---
Financing Agreements [Line Items] |  |  |
Unpaid Principal Balance | $ 3,998,346 | $ 4,339,479 |
Fair value / Carrying Value | 3,995,982 | 4,336,976 |
Financing agreements with non-mark-to-market collateral provisions |  |  |
Financing Agreements [Line Items] |  |  |
Unpaid Principal Balance | 1,039,205 | 1,156,899 |
Amortized Cost Balance | 1,039,205 | 1,156,899 |
Fair value / Carrying Value | 1,041,283 | 1,159,213 |
Financing agreements with mark-to-market collateral provisions |  |  |
Financing Agreements [Line Items] |  |  |
Unpaid Principal Balance | 1,180,287 | 1,338,077 |
Amortized Cost Balance | 1,180,287 | 1,338,077 |
Fair value / Carrying Value | 1,180,287 | 1,338,077 | $ 1,650,000
Securitized debt |  |  |
Financing Agreements [Line Items] |  |  |
Unpaid Principal Balance | 748,717 | 866,203 |
Amortized Cost Balance | 748,708 | 857,553 |
Fair value / Carrying Value | 753,008 | 869,482 |
Total Financing agreements, at fair value |  |  |
Financing Agreements [Line Items] |  |  |
Unpaid Principal Balance | 2,968,209 | 3,361,179 |
Amortized Cost Balance | 2,968,200 | 3,352,529 |
Fair value / Carrying Value | 2,974,578 | 3,366,772 |
Securitized debt |  |  |
Financing Agreements [Line Items] |  |  |
Unpaid Principal Balance | 800,137 | 648,300 |
Fair value / Carrying Value | 795,912 | 645,027 |
Convertible senior notes |  |  |
Financing Agreements [Line Items] |  |  |
Unpaid Principal Balance | 230,000 | 230,000 |
Fair value / Carrying Value | 225,492 | 225,177 |
Senior notes |  |  |
Financing Agreements [Line Items] |  |  |
Unpaid Principal Balance |  | 100,000 |
Fair value / Carrying Value |  | 100,000 |
Total Financing agreements at carrying value |  |  |
Financing Agreements [Line Items] |  |  |
Unpaid Principal Balance | 1,030,137 | 978,300 |
Fair value / Carrying Value | $ 1,021,404 | $ 970,204 |

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