Mortgage-Backed Securities - Year Originated (Detail) - Non-agency MBS	Mar. 31, 2017	Dec. 31, 2016
Origination Year as a Percentage of Outstanding Principal Balance:
2003 (and prior)	3.60%	3.60%
2004	4.20%	4.20%
2005	20.40%	20.20%
2006	37.70%	38.00%
2007	31.50%	31.30%
2008	1.80%	1.80%
2009 and later	0.80%	0.90%
Total	100.00%	100.00%