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Financing Agreements (Financing Agreements) (Details) - USD ($) $ in Thousands | Jun. 30, 2021 | Dec. 31, 2020
--- | --- | ---
Financing Agreements [Line Items] |  |
Unpaid Principal Balance | $ 4,433,769 | $ 4,339,479
Fair value / Carrying Value | 4,428,791 | 4,336,976
Fair Value |  |
Financing Agreements [Line Items] |  |
Unpaid Principal Balance | 2,287,990 | 3,361,179
Amortized Cost Balance | 2,287,983 | 3,352,529
Fair value / Carrying Value | 2,294,103 | 3,366,772
Carrying Value |  |
Financing Agreements [Line Items] |  |
Unpaid Principal Balance | 2,145,779 | 978,300
Fair value / Carrying Value | 2,134,688 | 970,204
Financing agreements with non-mark-to-market collateral provisions | Fair Value |  |
Financing Agreements [Line Items] |  |
Unpaid Principal Balance | 793,754 | 1,156,899
Amortized Cost Balance | 793,754 | 1,156,899
Fair value / Carrying Value | 795,341 | 1,159,213
Agreements with mark-to-market collateral provisions |  |
Financing Agreements [Line Items] |  |
Fair value / Carrying Value | 858,100 |
Agreements with mark-to-market collateral provisions | Fair Value |  |
Financing Agreements [Line Items] |  |
Unpaid Principal Balance | 858,066 | 1,338,077
Amortized Cost Balance | 858,066 | 1,338,077
Fair value / Carrying Value | 858,066 | 1,338,077
Agreements with mark-to-market collateral provisions | Carrying Value |  |
Financing Agreements [Line Items] |  |
Unpaid Principal Balance | 503,303 |
Fair value / Carrying Value | 503,191 |
Securitized debt | Fair Value |  |
Financing Agreements [Line Items] |  |
Unpaid Principal Balance | 636,170 | 866,203
Amortized Cost Balance | 636,163 | 857,553
Fair value / Carrying Value | 640,696 | 869,482
Securitized debt | Carrying Value |  |
Financing Agreements [Line Items] |  |
Unpaid Principal Balance | 1,412,476 | 648,300
Fair value / Carrying Value | 1,405,685 | 645,027
Convertible senior notes | Carrying Value |  |
Financing Agreements [Line Items] |  |
Unpaid Principal Balance | 230,000 | 230,000
Fair value / Carrying Value | $ 225,812 | 225,177
Senior notes | Carrying Value |  |
Financing Agreements [Line Items] |  |
Unpaid Principal Balance |  | 100,000
Fair value / Carrying Value |  | $ 100,000

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