Mortgage-Backed Securities - Year Originated (Detail) - Non-agency MBS	Jun. 30, 2019	Dec. 31, 2018
Origination Year as a Percentage of Outstanding Principal Balance:
2003 (and prior)	1.40%	1.30%
2004	1.60%	1.70%
2005	11.50%	12.90%
2006	52.90%	49.80%
2007	27.30%	31.00%
2008 and later	5.30%	3.30%
Total	100.00%	100.00%