Use of Special Purpose Entities and Variable Interest Entities (Loan Securitization Transaction) (Details) - USD ($) $ in Thousands	12 Months Ended
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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