Residential Mortgage-Backed Securities - Year Originated (Detail) (Non-Agency RMBS [Member])	Sep. 30, 2012	Dec. 31, 2011
Non-Agency RMBS [Member]
Origination Year as a Percentage of Outstanding Principal Balance
2001	0.20%	0.20%
2003	0.40%	0.90%
2004	0.50%	1.30%
2005	13.80%	13.80%
2006	32.20%	31.80%
2007	49.90%	48.70%
2008	3.00%	3.30%
Total	100.00%	100.00%