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Financing Agreements (Financing Agreements) (Details) - USD ($) $ in Thousands | Sep. 30, 2021 | Dec. 31, 2020
--- | --- | ---
Financing Agreements [Line Items] |  |
Unpaid Principal Balance | $ 5,557,639 | $ 4,339,479
Fair value / Carrying Value | 5,550,808 | 4,336,976
Fair Value |  |
Financing Agreements [Line Items] |  |
Unpaid Principal Balance | 2,491,537 | 3,361,179
Amortized Cost Balance | 2,491,531 | 3,352,529
Fair value / Carrying Value | 2,496,584 | 3,366,772
Carrying Value |  |
Financing Agreements [Line Items] |  |
Unpaid Principal Balance | 3,066,102 | 978,300
Fair value / Carrying Value | 3,054,224 | 970,204
Financing agreements with non-mark-to-market collateral provisions | Fair Value |  |
Financing Agreements [Line Items] |  |
Unpaid Principal Balance | 689,205 | 1,156,899
Amortized Cost Balance | 689,205 | 1,156,899
Fair value / Carrying Value | 690,583 | 1,159,213
Financing agreements with non-mark-to-market collateral provisions | Carrying Value |  |
Financing Agreements [Line Items] |  |
Unpaid Principal Balance | 157,784 |
Fair value / Carrying Value | 157,366 |
Agreements with mark-to-market collateral provisions |  |
Financing Agreements [Line Items] |  |
Fair value / Carrying Value | 1,300,000 |
Agreements with mark-to-market collateral provisions | Fair Value |  |
Financing Agreements [Line Items] |  |
Unpaid Principal Balance | 1,275,172 | 1,338,077
Amortized Cost Balance | 1,275,172 | 1,338,077
Fair value / Carrying Value | 1,275,172 | 1,338,077
Agreements with mark-to-market collateral provisions | Carrying Value |  |
Financing Agreements [Line Items] |  |
Unpaid Principal Balance | 1,155,964 |
Fair value / Carrying Value | 1,155,820 |
Securitized debt | Fair Value |  |
Financing Agreements [Line Items] |  |
Unpaid Principal Balance | 527,160 | 866,203
Amortized Cost Balance | 527,154 | 857,553
Fair value / Carrying Value | 530,829 | 869,482
Securitized debt | Carrying Value |  |
Financing Agreements [Line Items] |  |
Unpaid Principal Balance | 1,522,354 | 648,300
Fair value / Carrying Value | 1,514,900 | 645,027
Convertible senior notes | Carrying Value |  |
Financing Agreements [Line Items] |  |
Unpaid Principal Balance | 230,000 | 230,000
Fair value / Carrying Value | $ 226,138 | 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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