Loans Held for Investment - Percentage of Securitized Loans Held For Investment Carried at Fair Value by Year Originated (Details) - Loans Held for Investment at Fair Value - Seasoned Subprime Residential Mortgage Loans - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Securitized Loans Held For Investment By Consolidated Variable Interest Entities [Line Items]
2002 and prior	6.50%	6.90%
2003	5.70%	5.90%
2004	11.80%	12.00%
2005	18.70%	18.20%
2006	22.80%	22.70%
2007	22.20%	22.40%
2008	6.20%	6.40%
2009	1.20%	1.30%
2010 and later	4.90%	4.20%
Total	100.00%	100.00%
Underlying mortgages excluded from portfolio	$ 148