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Use of Special Purpose Entities and Variable Interest Entities (Loan Securitization Transactions) (Details) - USD ($) $ in Thousands | 1 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2017 | Dec. 31, 2019 | Dec. 31, 2018
Variable Interest Entity [Line Items] |  |  |
Securitized debt |  | $ 570,952 | $ 684,420
Asset-backed Securities, Securitized Loans and Receivables |  |  |
Variable Interest Entity [Line Items] |  |  |
Unpaid principal balance residential whole loans sold |  | 1,290,029 | 1,290,029
Securitized debt |  | $ 570,952 | $ 684,420
Weighted average interest rate |  | 3.68% | 3.66%
Weighted average contractual maturity of Senior Bonds |  | 30 years | 31 years
Cash received |  | $ 802,815 | $ 802,815
Debt issuance cost |  | $ 2,900 | 3,800
Debt instrument, basis spread on variable rate |  | 3.00% |
Debt instrument, coupon step-up period | 36 months |  |
Asset-backed Securities, Securitized Loans and Receivables | Senior Notes |  |  |
Variable Interest Entity [Line Items] |  |  |
Face amount |  | $ 802,817 | 802,817
Proceeds from senior bonds sold with step-up feature |  | 493,200 | 582,800
Asset-backed Securities, Securitized Loans and Receivables | Rated and Non-Rated Certificates |  |  |
Variable Interest Entity [Line Items] |  |  |
Face amount |  | $ 275,174 | $ 275,174

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