Mortgage-Backed Securities - Percentage of Non-Agency RMBS By Year Originated (Details) - Non-Agency RMBS	Dec. 31, 2020	Dec. 31, 2019
Origination Year as a Percentage of Outstanding Principal Balance:
2003 (and prior)	1.70%	1.30%
2004	1.40%	1.50%
2005	10.30%	10.70%
2006	52.10%	52.90%
2007	27.20%	26.60%
2008 and later	7.30%	7.00%
Total	100.00%	100.00%