Mortgage-Backed Securities - Year Originated (Detail) - Non-Agency RMBS	Dec. 31, 2019	Dec. 31, 2018
Origination Year as a Percentage of Outstanding Principal Balance:
2003 (and prior)	1.30%	1.30%
2004	1.50%	1.70%
2005	10.70%	12.90%
2006	52.90%	49.80%
2007	26.60%	31.00%
2008 and later	7.00%	3.30%
Total	100.00%	100.00%