Mortgage-Backed Securities - Percentage of Non-Agency RMBS By Year Originated (Details) - Non-Agency RMBS	Sep. 30, 2021	Dec. 31, 2020
Origination Year as a Percentage of Outstanding Principal Balance:
2003 (and prior)	1.40%	1.70%
2004	1.30%	1.40%
2005	9.10%	10.30%
2006	53.00%	52.10%
2007	25.60%	27.20%
2008 and later	9.60%	7.30%
Total	100.00%	100.00%