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Use of Special Purpose Entities and Variable Interest Entities (Loan Securitization Transaction) (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019
Asset-backed Securities, Securitized Loans and Receivables |  |
Variable Interest Entity [Line Items] |  |
Aggregate fair value | $ 2,232,561 | $ 1,290,029
Outstanding amount of Senior Bonds, at carrying value | 645,027 | 570,952
Outstanding amount of Senior Bonds, at fair value | 869,482 | 0
Outstanding amount of Senior Bonds, total | $ 1,514,509 | $ 570,952
Weighted average interest rate | 2.11% | 3.68%
Weighted average contractual maturity of Senior Bonds | 41 years | 30 years
Cash received | $ 1,853,408 | $ 802,815
Debt issuance cost | 3,200 | 2,900
Senior Notes |  |
Variable Interest Entity [Line Items] |  |
Cash received | 1,300,000 |
Senior Notes | Asset-backed Securities, Securitized Loans and Receivables |  |
Variable Interest Entity [Line Items] |  |
Debt instrument, face amount | 1,862,068 | 802,817
Proceeds from Senior bond sold with Step up feature | $ 568,700 | 493,200
Debt instrument, coupon step-up period | 36 months |
Senior Notes | Asset-backed Securities, Securitized Loans and Receivables | Minimum |  |
Variable Interest Entity [Line Items] |  |
Debt instrument, basis spread on variable rate | 1.00% |
Senior Notes | Asset-backed Securities, Securitized Loans and Receivables | Maximum |  |
Variable Interest Entity [Line Items] |  |
Debt instrument, basis spread on variable rate | 3.00% |
Senior Support Certificates | Asset-backed Securities, Securitized Loans and Receivables |  |
Variable Interest Entity [Line Items] |  |
Debt instrument, face amount | $ 268,548 | $ 275,174

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