Use of Special Purpose Entities and Variable Interest Entities - Loan Securitization Transaction (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2022	Dec. 31, 2021
Asset-backed Securities, Securitized Loans and Receivables
Variable Interest Entity [Line Items]
Aggregate unpaid principal balance of residential whole loans sold	$ 4,554,495	$ 3,984,355
Outstanding amount of Senior Bonds, at carrying value	1,173,265	1,334,342
Outstanding amount of Senior Bonds, at fair value	1,685,796	1,316,131
Outstanding amount of Senior Bonds, total	$ 2,859,061	$ 2,650,473
Weighted average interest rate	2.41%	2.01%
Weighted average contractual maturity of Senior Bonds	38 years	36 years
Cash received	$ 4,193,362	$ 3,682,082
Debt issuance cost	5,600	6,800
Senior Notes
Variable Interest Entity [Line Items]
Cash received	513,900
Senior Notes \| Asset-backed Securities, Securitized Loans and Receivables
Variable Interest Entity [Line Items]
Debt instrument, face amount	4,181,681	3,667,790
Proceeds from Senior bond sold with Step up feature	$ 707,600	329,000
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	$ 339,975	$ 283,930

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